Surface-emission laser diode and fabrication process thereof

ABSTRACT

A surface-emission laser diode includes a GaAs substrate, a cavity region, and upper and lower reflectors provided at a top part and a bottom part of the cavity region, the upper reflector and/or the lower reflector including a semiconductor Bragg reflector, at least a part of the semiconductor distributed Bragg reflector includes a semiconductor layer containing Al, Ga and As as major components, there being provided, between the active layer and the semiconductor layer that contains Al, Ga and As as major components, a semiconductor layer containing Al, In and P as major components adjacent to the semiconductor layer that contains Al, Ga and As as major components, with an interface formed coincident to a location of a node of electric strength distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Rule 1.53(b) continuation of U.S. patentapplication Ser. No. 12/691,476 filed Jan. 21, 2010 now U.S. Pat. No.8,199,788, which in turn is a continuation of U.S. patent applicationSer. No. 10/567,809, filed Apr. 25, 2008 as a Section 371 national stageof PCT/2005/010520 filed Jun. 8, 2005, now U.S. Pat. No. 7,684,458,which claims the priority of Japanese patent applications nos.2004-173890, 2004-159671, 2005-088188, and 2005-101765 filed with theJapanese Patent Office on Jun. 11, 2004, Dec. 13, 2004, Mar. 25, 2005,and Mar. 31, 2005, respectively.

TECHNICAL FIELD

The present invention relates to surface-emission laser diodes and thefabrication process thereof.

BACKGROUND ART

A surface-emission laser diode (surface-emission semiconductor laser) isa laser diode that emits light in a vertical direction to the substrateand is used for civil purposes, such as optical source of opticaltelecommunication including optical interconnection, optical source ofoptical pickup devices, optical source of image forming apparatuses, andthe like.

-   Patent Reference 1 Japanese Laid-Open Patent Application 2002-164621-   Patent Reference 2 Japanese Laid-Open Patent Application 9-107153    official gazette-   Patent Reference 3 Japanese Laid-Open Patent Application 2001-60739-   Patent Reference 4 Japanese Laid Open Patent Application 2001-168461-   Non-patent Reference 1 IEEE Photonics Technology Letters, Vol, 10,    No. 12, pp. 1676-1678, 1998 (Tokyo Institute of Technology)-   Non-patent Reference 2 IEEE Photonics Technology Letters, Vol. 12,    No. 6, pp. 603-605, 2000 (Wisconsin Univ.)

DISCLOSURE OF THE INVENTION

With a surface-emission laser diode for such applications, it isrequired that the laser diode has a large gain for the active layer, lowthreshold value, high output, excellent reliability, and controlledpolarization direction.

Particularly, surface-emission laser diodes have a tendency of providingsmall optical output as compared with edge-emission laser diodes in viewof the small volume of the active layer. Thus, there are many cases inwhich demand of increased output is imposed to such surface-emissionlaser diodes. One approach of increasing the optical output is tosuppress the temperature rise of the optical emission part.

First, explanation will be made for a typical device structure ofsurface-emission laser diode and description will be made further withregard to the measures taken against heat.

FIG. 1 is a diagram showing an example structure of generalsurface-emission laser diode. It should be noted that the example ofFIG. 1 is a surface-emission laser diode having InGaAs/GaAs for theactive layer and using the current confinement structure formed byselective oxidization of an AlAs layer.

The general surface-emission laser diode shown in FIG. 1 is produced inthe following manner.

By an MOCVD process or MBE process, there is formed a stack of films byconsecutively stacking, on an n-GaAs single crystal substrate 11, ann-AlGaAs/n-AlGaAs lower semiconductor multilayer reflector (DBR:distributed Bragg reflector) 12, a lower GaAs spacer layer 13, anAlGaAs/AlGaAs-MQW active layer 14, an upper AlGaAs spacer layer 15, anAlAs layer for selective oxidation 16, and p-AlGaAs/p-AlGaAs uppersemiconductor multilayer reflector (DBR).

Next, the stacked film is processed to form a mesa structure by a dryetching process. With this process, it is commonly practiced to conductthe etching such that the etching surface reaches a region inside thelower semiconductor DBR 12.

Next, the AlAs layer for selective oxidation 16 having the sidewallsurface exposed by the dry etching process is processed in water vapor,such that there occurs oxidation at the peripheral part thereof, to forman insulation layer of Al_(x)O_(y) in such a peripheral part. Thereby,there is formed a current confinement structure that restricts the pathof the device drive current only to the AlAs region at the central partwhere the oxidation has not taken place.

Next, the peripheral part of the mesa structure is filled with aninsulation film 18, and a p-side electrode 19 and an n-side electrode 20are formed at predetermined locations. With this, fabrication of thesurface-emission laser diode of FIG. 1 is completed.

Meanwhile, with such a surface-emission laser diode, it is preferable toreduce the height of the mesa structure and thereby to reduce the heatresistance to the substrate for the purpose of improving heatdissipation from the active layer 14. However, with the mesa formationprocess conducted by a dry etching process, fluctuation in the etchingdepth of ±10% or more is not unusual, and further in view of thepossible fluctuation of etching depth inside the specimen, it isinevitable to set the height of the mesa structure to be larger than theheight which is actually necessary.

Patent Reference 1 shows the construction that reduces heat resistance.

With the construction shown in Patent Reference 1, AlAs, having a muchlarger thermal conductivity as compared with AlGaAs, is used for the lowrefractive index layers in the majority part of the DBR located at thelower part of the lower semiconductor DBR. On the other hand,conventional AlGaAs is used for the low refractive index layer in theupper part of lower semiconductor DBR. The reason of this is to avoidthe situation that the etching surface reaches inside the lower partAlAs-DBR and there takes place oxidation at the edge surface of the AlAslayer of lower part AlAs-DBR exposed at the mesa sidewall surface at thetime of conducting the oxidation processing of the AlAs layer forselective oxidation later. When this occurs, the active part in thedevice is insulated or there occurs increase of device resistance. Thereason that such a situation arises is that the oxidation rate of AlGaAsrelies heavily upon the Al content and that there occurs increase in theoxidation rate with increase of the Al content. Thus, the oxidation ratebecomes the largest with AlAs.

In order to avoid this problem, Patent Reference 1 forms the upperAlGaAs-DBR by using AlGaAs of small oxidation rate and controls thebottom surface of etching such that the bottom surface of etching islocated in this upper AlGaAs-DBR. Thereby, exposure of the AlAs edgesurface of the lower AlAs-DBR is avoided. With such an upper AlGaAs-DBR,it is preferable that the pair number is equal to or smaller than 4/7 ofthe entire pair number as set forth in Patent Reference 1. Particularly,it is desirable to set the pair number to 10 pairs or less.

However, with the approach of controlling the bottom surface of etchingsuch that the bottom surface of etching is located in the upperAlGaAs-DBR of only about 10 pairs by way of controlling the etchingtime, there is caused remarkable decrease of yield, and there arises aproblem in that large fluctuation occurs in the position of the bottomsurface of etching in the upper AlGaAs-DBR.

Therefore, in order to achieve the object of providing measures againstheat while maintaining high yield, control of the mesa formation processby dry etching process becomes important. For this, it is desirable tocarry out monitoring of the etching process.

For the monitoring method of dry etching process, there is a knownmethod of plasma atomic emission spectrometry. Further, there is amethod that irradiates the surface of the specimen to be etched withlight, wherein this method monitors the intensity of the reflectionlight and detects the etching depth from the change of the interferenceintensity of the film. From the viewpoint that there is no need ofproviding observation window and in view of the fact that the method iswell established and that there are commercially available instruments,it is thought advantageous to use the plasma atomic emissionspectrometry.

With the monitoring method that uses this plasma atomic emissionspectrometry, the change of emission intensity corresponding to thesemiconductor film composition is detected by monitoring the atomicemission intensity of Ga at 417 nm or the time change of the ratio ofthe atomic emission intensity of Ga at 417 nm to the atomic emissionintensity of Al at 396 nm. In the case of the specimen formed primarilyof repetition of the layers of two compositions as in the case of thelayered structure of the surface-emission laser diode, this atomicemission intensity change draws an oscillatory waveform.

However, in the semiconductor film used for the surface-emission laserdiode that oscillates at the laser oscillation wavelength of a GaAsactive layer (about 850 nm), in particular, the change of Ga compositionor Ga/Al composition ratio is small. Further, the film thickness of theDBR or cavity is small in correspondence to the wavelength. Thus, theamplitude of the oscillatory waveform of the plasma atomic emissionspectrometry becomes small. Thereby, it is not easy to carry out themonitoring. Further, in the case the specimen to be etched has largesize, there arises a problem in that monitoring becomes difficultbecause of the distribution of etching rate inside the specimen.

Meanwhile, it is known that excellent carrier confinement to the activelayer is attained in the surface-emission laser diode of the 850 nm bandand 980 nm band.

For example, GaAs is used for the quantum well active layer and AlGaAsis used for the barrier layer and the spacer layer (cladding layer) of asurface-emission laser diode of the 850 nm band. Further, with such asurface-emission laser diode of the 850 nm band, it is possible to usethe current confinement structure that uses a high-performanceAlGaAs-system reflector (DBR) and the current confinement structure thatuses an Al oxide film.

Further, various proposals have been made with regard to polarizationcontrol of such a surface-emission laser diode. For example, there isproposed a method of providing anisotropy in the outer shape of theactive layer as viewed from the optical emission direction.Particularly, Non-Patent Reference 1 shows that polarization control ispossible by anisotropy of optical gain realized by using a (311)Bsubstrate, in other words a so-called off-substrate that is inclinedfrom (100) by 25° in the (111)B direction, such that the optical gain inthe inclined direction is increased. Further, it is shown that similareffect is obtained also with a (311)A substrate.

However, with the technology of Non-Patent Reference 1, there is adrawback in that it is difficult to conduct crystal growth on theheavily inclined (311)B substrate as compared with the crystal growth onthe (100) substrate and that crystal growth on the (311)A substrate iseven more difficult.

Further, in any of these substrates in which the substrate is heavilyinclined, the cost of the substrate is increased by twice or more and itis difficult to carry out cleaving process. Further, handling of thesubstrate is difficult.

Meanwhile, the surface-emission laser diode of the wavelength shorterthan 850 nm is realized by increasing the bandgap of the quantum wellactive layer by adding thereto Al.

For example, there is a proposal of a surface-emission laser diode ofthe 780 nm band in which Al is added to the quantum well active layer byabout 12% in terms of the compositional ratio.

However, such a surface emission laser diode of the band shorter than850 nm, there is caused a decrease in the band discontinuity between thequantum well active layer and the barrier layer or the spacer layer, andthe efficiency of carrier confinement to the active layer is decreased.Thereby, there arises a problem in that it is difficult to attain goodtemperature characteristics as compared with the surface-emission laserdiode of the 850 nm band. Further, because the active layer is addedwith active Al, there is a tendency that oxygen is incorporated into theactive layer during the growth or processing thereof, while this leadsto the problem of formation of non-optical recombination center, whichin turn invites degradation of efficiency of optical emission anddegradation of reliability.

Patent Reference 2 proposes a surface-emission laser diode (780 nm band)that uses an Al-free active region (quantum well active layer and thelayers adjacent thereto) in a surface-emission laser diode of thewavelength band of 850 nm or shorter for the purpose of suppressing theformation of non-optical recombination center.

With this conventional surface-emission laser diode, GaAsP having atensile strain is used for the quantum well active layer, and GaInPhaving a compressive strain is used for the barrier layer. Further,lattice matched GaInP is used for the spacer layer (the layer betweenthe cladding layer and the first and third quantum well active layer),and AlGaInP is used for the cladding layer. According to the technologyof Patent Reference 2, reliability of the surface-emission laser diodeis improved in view of use of the Al-free composition of the activeregion.

Further, Non-Patent Reference 2 proposes a surface-emission laser diodeof the 780 nm band that uses GaInPAs having a compressive strain for thequantum well active layer for the purpose of attaining the effect ofAl-free active region and further for the purpose of increasing the gainof the active layer, wherein Non-Patent Reference 2 further teaches theuse of lattice matched GaInP or GaInP having a tensile strain for thebarrier layer and the use of lattice matched AlGaInP for the spacerlayer (the layer between the cladding layer and the first and thirdquantum well active layers) and further the use of AlGaInP (having Alcomposition larger than the spacer layer) for the cladding layer.

With the surface-emission laser diode of Non-Patent Reference 2, thebarrier layer has a lattice matched composition, and thus, the barrierlayer has a larger bandgap as compared with the compressive straincomposition, and thus, the efficiency of carrier confinement is improvedas compared with the structure of Patent Reference 2 mentioned before.

With regard to polarization control, Patent Reference 3 shows thetechnology of using a substrate having a surface orientation inclined inthe direction of (111)A surface or (111)B surface from the (100) surfaceorientation by the angle (inclination angle) of 15-40°, wherein thistechnology uses the anisotropy of optical gain in combination with themultiple quantum well active layer of InAlGaAs and InGaAsP having acompressive strain for increasing the optical gain in the inclineddirection.

Further, Patent Reference 4 shows the method that forms the mesa shapein a circular, elliptical or elongated circular shape and sets thedirection of the major axis in the direction of (111)A surface or (111)Bsurface from (100). In this case, a substrate having a surfaceorientation offset by 2° (including −5′-+5°) in the [110] direction from(100) is used. It should be noted that this is not the substrateinclined in the A surface or B surface direction.

However, it has not been realized a surface-emission laser diode of thewavelength shorter than 850 nm and at the same time having a large gainfor the active layer and small threshold value, high output power,excellent reliability and controlled polarization direction.

Thus, while Patent Reference 2 can provide improved reliability in viewof the use of Al-free active layer, the reference is silent about thecontrol method of polarization. Further, while Non-Patent Reference 2provides a structure of excellent carrier confinement, it is silentabout the control method of polarization. Further, while PatentReference 3 enables control of polarization direction, it is totallyindifferent to reliability or structure matched to the property of thematerials. Further, while Patent Reference 4 can control thepolarization direction, it is totally indifferent about achieving highgain and long lifetime for the surface-emission laser diode of thewavelength shorter than 650 nm.

Further, in the case a material of (Al)GaInP system is used for thematerial forming the cavity region sandwiched by the upper and lowerreflectors as described in Non-Patent Reference 2, it is known thatthere arises large increase of threshold current because of separation(segregation) of In such as carry-over of In into the AlGaAs layer atthe interface between the cavity region and the upper reflector, whichis formed by the material of the AlGaAs system.

Further, AlGaInP, a quaternary mixed crystal, has large thermalresistance, and there also arises a problem with a material of the(Al)GaInP system in that Zn (zinc), which is used for the p-type dopant,easily causes diffusion.

Thus, it has not been realized conventionally to provide asurface-emission laser diode of the wavelength shorter than 850 nm,having large gain for the active layer and low threshold value, highoutput, excellent reliability and controlled polarization direction.

Thus, it is the object of the present invention to provide, in asurface-emission laser diode having small Ga content or small change ofGa/Al ratio in a semiconductor distributed Bragg reflector (DBR) andhaving improved heat dissipation, a construction that can improve thecontrollability of etching at the time of forming the mesa structure byetching a laser stacking structure, and to provide a surface-emissionlaser diode having such a construction and capable of performing highoutput operation.

In addition, the present invention has an object of providing, in asurface-emission laser diode of the wavelength shorter than 850 nm, thesurface-emission laser diode of excellent reliability and a high-outputsurface-emission laser diode of low threshold value having a large gainof active layer.

Further, the present invention has an object of providing, in asurface-emission laser diode of the wavelength shorter than 850 nm, thesurface-emission laser diode of excellent reliability, having large gainfor the active layer and low threshold value, high output power andcontrolled polarization direction.

Further, the present invention provides a surface-emission laser diodearray, an image forming apparatus, an optical pickup system, an opticaltransmission module, an optical transceiver module and an opticaltelecommunication system in which the foregoing surface-emission laserdiode is integrated.

In a first aspect, the present invention provides a surface-emissionlaser diode, comprising:

a semiconductor substrate;

a cavity region formed over said semiconductor substrate, said cavityregion comprising: an active layer structural part including at leastone quantum well active layer producing a laser light and a barrierlayer; and a spacer layer provided in a vicinity of said active layerstructural part, said spacer layer comprising at least one material; and

an upper reflector and a lower reflector provided over saidsemiconductor substrate respectively at a top part and a bottom part ofsaid cavity region,

said cavity region, said upper reflector and said lower reflectorforming a mesa structure over said semiconductor substrate,

said upper reflector and said lower reflector constituting asemiconductor distributed Bragg reflector having a periodic change ofrefractive index and reflecting an incident light by interference ofoptical waves,

at least a part of said semiconductor distributed Bragg reflector beingformed of a layer of small refractive index of Al_(x)Ga_(1-x)As (0<x≦1)and a layer of large refractive index of Al_(y)Ga_(1-y)As

said lower reflector being formed of a first lower reflector having alow-refractive index layer of AlAs and a second lower reflector formedon said first lower reflector, said second lower reflector having alow-refractive index layer of AlGaAs,

wherein any one layer constituting said cavity region contains In.

In a second aspect, the present invention provides a surface-emissionlaser diode, comprising:

a (100) GaAs substrate having a surface orientation inclined in adirection of a (111)A surface by an angle of 5° to 20°;

a cavity region provided over said GaAs substrate, said cavity regionincluding an active layer structural part comprising at least one layerof quantum well active layer producing a laser light and barrier layers,and a spacer layer provided in a vicinity of said active layerstructural part, said spacer layer comprising at lease one material; and

an upper reflector and a lower reflector provided at a top part and abottom part of said cavity region,

said cavity region and said upper and lower reflectors forming a mesastructure over said GaAs substrate,

said upper reflector and said lower reflector comprising a semiconductordistributed Bragg reflector having a periodic change of refractive indexand reflecting an incident light by interference of optical waves,

at least a part of said semiconductor distributed Bragg reflector beingformed of a layer of small refractive index of Al_(x)Ga_(1-x)As (0<x≦1)and a layer of large refractive index of Al_(y)Ga_(1-y)As (0≦y<x≦1),

a part of said spacer layer comprising (Al_(a)Ga_(1-a))_(b)In_(1-b)P(0<a≦1, 0≦b≦1),

said quantum well active layer comprising Ga_(c)In_(1-c)P_(d)As_(1-d)(0≦c≦1, 0≦d≦1),

said barrier layers comprising Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1,0≦f≦1),

said quantum well active layer having a compressive strain,

said active layer structural part having a shape anisotropy elongated ina direction of a (111)A surface as viewed from a direction of lightemission.

In a third aspect, the present invention provides a method offabricating a surface emission laser diode, said surface emission layerdiode comprising, over a semiconductor substrate: a cavity regioncomprising an active layer structural part including at least onequantum well active layer producing a laser light and barrier layers,and a spacer layer of at least one material provided in a vicinity ofsaid active layer structural part; and an upper reflector and a lowerreflector provided at a top part and a bottom part of said cavityregion, said method comprising the steps of:

forming a stacked structure including said lower reflector, said cavityregion and said upper reflector over said semiconductor substrate; and

forming a mesa structure by patterning said stacked film by dry etching,

said step of forming said stacked structure including a step ofincorporating In to any one layer constituting said cavity region,

said step of forming said mesa structure by said dry etching comprises astep of controlling a height of said mesa structure by monitoring lightemission of In.

In a forth aspect, the present invention provides a surface-emissionlaser diode, comprising:

a GaAs substrate;

a cavity region formed over said GaAs substrate, said cavity regionincluding at least one quantum well active layer producing a laser lightand barrier layers; and

an upper reflector and a lower reflector provided at a top part and abottom part of said cavity region over said GaAs substrate,

said upper reflector and/or said lower reflector including asemiconductor Bragg reflector,

at least a part of said semiconductor distributed Bragg reflectorcomprising a semiconductor layer containing Al, Ga and As as majorcomponents,

wherein there is provided, between said active layer and saidsemiconductor layer that contains Al, Ga and As as major components, asemiconductor layer containing Al, In and P as major components suchthat said semiconductor layer containing Al, In and P as majorcomponents is provided adjacent to said semiconductor layer thatcontains Al, Ga and As as major components,

an interface between said semiconductor layer containing Al, Ga and Asas major components and said semiconductor layer containing Al, In and Pas major components being formed coincident to a location of a node ofelectric field strength distribution.

In a fifth aspect, the present invention provides a surface-emissionlaser diode, comprising:

a GaAs substrate;

a cavity region formed over said GaAs substrate and having at least onequantum well active layer producing a laser light and barrier layers;and

an upper reflector and a lower reflector provided at a top part and abottom part of said cavity region over said GaAs substrate,

said upper reflector and/or lower reflector including a semiconductordistributed Bragg reflector,

at least a part of said semiconductor distributed Bragg reflectorcomprising a semiconductor layer containing Al, Ga and As as majorcomponents,

there being provided, between said active layer and said semiconductorlayer containing Al, Ga and As as major components, a(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer adjacent to saidsemiconductor layer containing Al, Ga and As as major components,

said (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1), layer being addedwith Mg (magnesium) as a p-type dopant,

said semiconductor layer containing Al, Ga and As as major componentsbeing added with C (carbon) as a p-type dopant.

In a sixth aspect, the present invention provides a surface-emissionlaser diode, comprising:

a GaAs substrate;

a cavity region formed over said GaAs substrate, said cavity regionincluding at least one quantum well active layer producing a laser lightand barrier layers; and

an upper reflector and a lower reflector provided at a top part and abottom part of said cavity region over said GaAs substrate,

said upper reflector and/or lower reflector including a semiconductordistributed Bragg reflector,

at least a part of said semiconductor distributed Bragg reflectorcomprising a semiconductor layer containing Al, Ga and As as majorcomponents,

there being provided, between said active layer and said semiconductorlayer containing Al, Ga and As as major components, a(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer adjacent to saidsemiconductor layer containing Al, Ga and As as major components,

said (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer being asemiconductor layer formed of a short period superlattice structure ofAlInP and GaInP.

In a seventh aspect, the present invention provides a surface-emissionlaser diode, comprising:

a GaAs substrate;

a cavity region formed over said GaAs substrate, said cavity regionincluding at least one quantum well active layer producing a laser lightand barrier layers; and

an upper reflector and a lower reflector provided at a top part and abottom part of said cavity region over said GaAs substrate,

said upper reflector and/or lower reflector including a semiconductordistributed Bragg reflector,

at least a part of said semiconductor distributed Bragg reflectorcomprising a low refractive index layer of Al_(x)Ga_(1-x)As (0<x≦1) anda high refractive index layer of Al_(y)Ga_(1-y)As (0≦y<x≦1),

one of said low refractive index layers constituting said upperreflector and/or said lower reflector and located closest to said activelayer comprising (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1),

an interface between said cavity region and said low refractive indexlayer of said upper reflector and/or said lower reflector locatedclosest to said active layer being coincident to an anti-node of anelectric strength distribution.

According to said first aspect of the present invention, any one layerconstituting the cavity region contains In, and thus, precision andreproducibility of mesa etching is improved by detecting exposure ofsaid In-containing layer at the time of the mesa etching of the laserlamination structure comprising the cavity and the upper and lowersemiconductor DBRs. Even in the case the lower semiconductor DBRcontains AlAs/(Al)GaAs-DBR of excellent heat dissipation, it is possibleto realize a construction in with the AlAs/(Al)GaAs-DBR is provided tothe neighbor of the cavity. With such a construction, temperature riseat the time of laser driving is suppressed and a high output powersurface-emission laser diode of excellent temperature characteristics isprovided. At the same time, it becomes possible to provide asurface-emission laser diode having excellent uniformity in the lasercharacteristics and characterized by excellent reproducibility in theprocessing and excellent yield.

Particularly, by incorporating In into the upper or lower spacer layerconstituting the active region and having much larger thickness ascompared with the active layer structural part, it becomes possible toform the mesa structure with further improved reproducibility andfurther improved precision, and with this, it becomes possible to form asurface-emission laser diode of further improved temperaturecharacteristics, higher output power and further improved uniformity inlaser characteristics, with higher reproducibility of processing andwith higher yield.

Further, by forming the semiconductor DBR constituting the second lowerreflector in the surface-emission laser diode according to the firstaspect, such that the semiconductor DBR has the thickness of 10 pairs orless, the thickness of the semiconductor DBR is set to be larger thanthe precision of the mesa etching and at the same time minimum. Withthis, temperature rise at the time of driving is suppressed further, anda surface-emission laser diode of high output power is obtained withexcellent temperature characteristics.

Further, by forming a part of the spacer layer of the surface-emissionlaser diode according to the first aspect of the present invention by(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) and by forming the quantumwell active layer by Ga_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1), andfurther by forming the barrier layer by Ga_(e)In_(1-e)P^(f)As_(1-f)(0≦e≦1, 0≦f≦1), and further by using an AlGaInP material and thus(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) for a part of the spacerlayer, it becomes possible to increase the bandgap difference betweenthe spacer layer and the quantum well active layer as compared with thecase of forming the spacer layer by the AlGaAs system. Thereby, there isachieved improvement of carrier confinement efficiency, and it becomespossible to realize a high-output laser having a further lower thresholdvalue in combination with the excellent heat dissipation effectpertinent to such a structure.

Further, with the surface emission laser diode according to the firstaspect of the present invention, it should be noted that a GaInPAsmaterial is used for the barrier layer and the quantum well activelayer, and thus, the quantum well active layer is formed ofGa_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1), and that the barrier layersare formed of Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1). Thus,according to the first feature of the present invention, the activelayer structural part formed of the quantum well active layer and thelayer adjacent thereto does not contain Al. With this, the problem ofincorporation of oxygen into the active layer structural part by Al, andassociated problem of formation of non-optical recombination center, issuppressed, and a surface-emission laser diode of long lifetime isrealized.

Further, by forming the quantum well active layer with a compressivestrain composition with the surface-emission laser diode according tothe first aspect such that a compressive strain is accumulated in thequantum well active layer, the carrier confinement effect is augmentedwith the effect of the strain, and the optical gain of the active layerstructural part is increased further. Further, improvement of heatdissipation is added to the foregoing. Thereby, the threshold value isdecreased further. Thus, it becomes possible to realize asurface-emission laser diode of extremely high efficiency and highoutput power. Further, with decrease of the threshold value attained bythe improvement of carrier confinement efficiency and further by theincrease of gain as a result of use of the strained quantum well activelayer, it becomes possible to decrease the reflectivity of the DBR atthe exit side of light (upper semiconductor DBR). As a result ofdecrease of the reflectivity of the DBR at the optical exit side, afurther increase of the optical power becomes possible.

Further, because the semiconductor substrate is a (100) GaAs substratehaving a surface orientation inclined in the direction of a (111)Asurface with an angle in the range from 5° to 20° (in other words, as aresult of use of the (100) GaAs substrate having the surface orientationinclined in the direction of the (111)A surface with the angle of 5° to20° by taking into consideration the surface orientation of thesubstrate) in the surface-emission laser diode according to the firstaspect of the present invention, adversary effects to the devicecharacteristics of the semiconductor layer, such as decrease of bandgapcaused by formation of natural super lattices or deterioration ofsurface morphology or formation of non-optical recombination centerscaused by hillock (hill-like defect), are reduced.

Further, with the surface-emission laser diode according to the firstaspect, which uses the (100) GaAs substrate having the surfaceorientation inclined in the direction of the (111)A surface within theangular range of 5° to 20° (in other words the (100)GaAs substratehaving the surface orientation inclined in the direction of the (111)Asurface within the angular range of 5° to 20° with regard to the controlof polarization), it is not possible to attain the polarization controleffect as in the case of using the (311)B substrate (corresponds to 25°inclination) currently drawing attention, and the anisotropy of opticalgain attained with the use of inclined substrate becomes inevitablysmall. With the surface-emission laser diode according to the firstaspect of the present invention, the foregoing decrease of anisotropy ofoptical gain can be compensated for by providing a compressive strain tothe quantum well active layer and induce increase of anisotropy of theoptical gain. Thereby, it becomes possible to control the polarizationdirection effectively. Thus, with the present invention, there isinduced a synergistic effect of improvement of heat dissipationefficiency and increase of gain of the active layer structural part, andit becomes possible to realize a high output power surface-emissionlaser diode oscillating at a wavelength shorter than 850 nm and at thesame time having low threshold value, excellent reliability andcontrolled polarization direction.

Next, according to the second aspect, with the use of the AlGaInPmaterial, and thus (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1), for apart of the space layer, it becomes possible to secure a large bandgapbetween the spacer layer and the quantum well active layer as comparedwith the case of forming the spacer layer with the AlGaAs system, andthe efficiency of carrier confinement is improved. Further, thethreshold of laser oscillation is decreased and it becomes possible toincrease the output.

With the surface-emission laser diode of the second aspect, too, theactive layer structural part formed of the quantum well layer and thelayer adjacent thereto has an Al-free construction as a result of use ofGaInPAs material for the barrier layers and the quantum well activelayer, and as a result, incorporation of oxygen into the active layerstructure by oxygen is reduced, and formation of the non-recombinationcenters is suppressed. With this, a surface-emission laser diode of longlifetime is realized.

Further, with the present invention, it becomes possible to decrease thethreshold value of laser oscillation further with the effect of thecompressive strain, by using a compressive strain composition for thequantum well active layer, and it becomes possible to increase theefficiency of laser oscillation and obtain large output.

Further, with the surface-emission laser diode of the second aspect,too, there is caused further decrease of threshold value of laseroscillation because of the improvement of efficiency of carrierconfinement to the active layer structural part and because ofimprovement of gain by the use of the strained quantum well activelayer, and it becomes possible to decrease the reflectivity of theexit-side DBR. Thereby, it becomes possible to obtain further higheroutput.

Further, with the surface-emission laser diode according to the secondaspect of the present invention, it becomes possible, with the use ofthe (100)GaAs substrate having a surface orientation inclined in thedirection of a (111)A surface with an angle within the range of 5° to20° by taking into consideration the effect of the surface orientationof the substrate, to suppress the adversary the effects to the devicecharacteristics of the laser such diode as decrease of bandgap caused byformation of natural super lattices, deterioration of surface morphologycaused by hillocks (hill-like defect), formation of non-opticalrecombination centers, or the like.

With the polarization control, the surface-emission laser diodeaccording to the second aspect of the present invention cannot utilizethe effect attained by using the (311)B substrate, which is through mostpromising at the present juncture as noted before, and the anisotropy ofoptical gain associated with the use of inclined substrate becomesinevitably small. With the present invention, this decrease iscompensated for, by increasing the anisotropy of optical gain attainedby providing a compressive strain to the quantum well active layer, andby increasing the optical gain in the inclined direction of thesubstrate (111)A surface direction) by providing anisotropy in the outershape of the active layer as viewed from the optical emission directionof the surface-emission laser diode. With this, control of polarizationdirection becomes extremely easy.

Thus, according to the surface-emission laser diode of the second aspectof the present invention, it becomes possible to realize a high outputpower surface-emission laser diode oscillating at the wavelength shorterthan 850 nm and having a large optical gain for the active layerstructural part, small threshold value of laser oscillation, excellentreliability, and controlled polarization plane.

Further, with the surface-emission laser diode of the second aspect ofthe present invention, it becomes possible to increase the banddiscontinuity to the quantum well active layer by accumulating a tensilestrain in the barrier layers. Thereby, it becomes possible to increasethe gain. With this, the threshold value of laser oscillation isdecreased and the surface-emission laser diode becomes possible toperform high output power operation. With the material of the GaInPAssystem, it should be noted that the semiconductor material constitutingthe barrier layer can increase the bandgap by decreasing the latticeconstant.

Further, according to the surface-emission laser diode of the secondaspect of the present invention, it becomes possible to realize asurface-emission laser diode of the oscillation wavelength larger thanabout 680 nm. Further, as a result of the use of the AlGaInP systemmaterial for the spacer layer, it becomes possible to realize thecarrier confinement equivalent to or superior to the case of thesurface-emission laser diode of the 780 nm band that uses the activelayer of the AlGaAs system, even in the case the active layer, formed ofthe quantum well layer and barrier layer, is formed of a material freefrom Al, as long as the compositional wavelength is 680 nm or longer.Further, the effect of the strained quantum well active layer is addedthereto. Thus, it becomes possible to realize the characteristicsequivalent to or superior to the surface-emission layer diode of the 780nm band that has the active layer of the AlGaAs system.

Further, according to the third aspect of the present invention thatprovides the method of fabricating a surface emission laser diode, saidsurface emission layer diode comprising, over a semiconductor substrate:a cavity region comprising an active layer structural part including atleast one quantum well active layer producing a laser light and barrierlayers, and a spacer layer of at least one material and provided in avicinity of said active layer structural part; and an upper reflectorand a lower reflector provided at a top part and a bottom part of saidcavity region, said method comprising the steps of: forming a stackedstructure including said lower reflector, said cavity region and saidupper reflector over said semiconductor substrate; and forming a mesastructure by patterning said stacked film by dry etching, said step offorming said stacked structure including a step of incorporating In toany one layer constituting said cavity region, said step of forming saidmesa structure by said dry etching comprises a step of controlling aheight of said mesa structure by monitoring light emission of In, itbecomes possible to detect the cavity part positively in the foregoingdry etching process, and with this, it becomes possible to form the mesastructure with good reproducibility and with excellent precision.According to the present invention, it becomes possible to fabricatesuch a surface-emission laser diode with good reproducibility and goodyield.

By providing, in a surface-emission laser diode according to the fourthor seventh aspects of the present invention, the interface between thesemiconductor layer containing Al, Ga and As as major components and thesemiconductor layer containing Al, In and P as major components to becoincident to a location of a node of electric field strengthdistribution, it becomes possible to decrease the effect of opticalabsorption at the foregoing interface significantly, even in the casethere is caused some segregation of In at the time of crystal growth ofthe semiconductor layer containing Al, Ga and As as the major componentson the semiconductor layer containing Al, In and P as the majorcomponents, and it becomes possible to suppress the adversary effect ofincrease of threshold value caused by the segregation of In.

By adding, in the surface-emission laser diode according to the fifth orseventh aspect, Mg (magnesium) to the semiconductor layer containing Al,In and P as the major components as a p-type dopant, and by adding C(carbon) to the semiconductor layer containing Al, Ga and As as themajor components as a p-type dopant, it becomes possible to suppress thediffusion of dopant and reduce the memory effect, and it becomespossible to carry out the doping with good controllability. Thereby, adoping profile near the designed profile is obtained, and degradation ofthe crystal quality of the active layer is suppressed. With this, a highoutput power surface-emission laser diode having a low threshold valuecan be realized easily.

Further, with the surface-emission laser diode according to the sixth orseventh aspects of the present invention, the efficiency of heatdissipation is improved and high output operation is realized easily bypseudo-constructing the AlGaInP mixed crystal by AlInP having smallthermal resistance and GaInP.

Further, according to the sixth and seventh aspects of the presentinvention, it becomes possible, with the user of the spacer layer of(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) in the surface-emissionlaser diode, to increase the bandgap difference between the spacer layerand the quantum well active layer as compared with the case of formingthe spacer layer by the AlGaAs system. Thereby, the threshold of laseroscillation is decreased, the efficiency of laser oscillation isimproved, and high output operation is realized. Further, by usingGa_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1) for the quantum well activelayer and by Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1) for the barrierlayers, it becomes possible to construct the active layer by a materialfree from Al, and the active region formed of the quantum well activelayer and the adjacent layer becomes Al-free. Thereby, it becomespossible to reduce the incorporation of oxygen, and formation of thenon-optical recombination centers is suppressed. Thereby, it becomespossible to realize a surface-emission laser diode of long lifetime.Thus, a high output power surface-emission laser diode of the wavelengthof 850 nm or shorter and having a large gain for the active layer, lowthreshold value of laser oscillation and good reliability is realized.

Further, according to the surface-emission laser diode of the sixth andseventh aspects of the present invention, it is possible to reduce thethreshold value of laser oscillation by the effect of strain, by usingthe compressive strain composition for the quantum well active layer,and the efficiency of laser oscillation is improved. Further, as aresult of improvement of the carrier confinement efficiency and increaseof the gain attained by the use of the strained quantum well activelayer, the threshold of laser oscillation is decreased further, and itbecomes possible to reduce the reflectivity of the exit-side DBR. Withthis, it becomes possible to increase the laser output further.

Further, according to the sixth and seventh aspects, it becomes possibleto increase the degree of freedom of design such as use of the quantumwell active layer of larger strain, by compensating for the strain ofthe quantum well active layer in the surface-emission laser diode.Further, because the material of the smaller lattice constant has alarger bandgap in the semiconductor material of the GaInPAs system andused for the barrier layer, it becomes possible to increase the banddiscontinuity to the quantum well active layer. Thereby, the gain isincreased and it becomes possible to carry out low-threshold valueoperation and high output power operation.

Further, according to the surface-emission laser diode of the seventhaspect, it becomes possible, by constructing the lower reflector suchthat the lower reflector includes AlAs having small thermal resistancefor the low refractive index layers, the dissipation characteristics ofheat generated in the active layer are improved, and the temperaturerise at the time of driving is suppressed. Thus, a high output powersurface-emission laser diode of excellent temperature characteristics isobtained.

Further, according to the surface-emission laser diode of the seventhaspect of the present invention, junction of the AlGaInP system materialand the AlGaAs system material is made easily by interposing anintermediate layer of small Al content between the low refractive indexlayer and the high refractive index layer of the semiconductordistributed Bragg reflector. Thus, in the case of laminating theAlyGa1−yAs (0≦y, x≦1) high refractive index layer on the(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive index layer,it becomes possible to conduct the growth of the high refractive indexlayer in a wide conditional range by reducing the Al content at theinterface. Further, with such a construction, the band discontinuity ofthe valence band is decreased and the resistance against the currentflowing in the stacked direction is reduced.

Further, according to the surface-emission laser diode of the sixth andseventh aspects, it becomes possible, as a result of the use of theAlGaInP system spacer layer, it becomes possible to realize the carrierconfinement equivalent to or superior to the case of thesurface-emission laser diode of the 780 nm band that uses the activelayer of the AlGaAs system, even in the case of using the Al-free activelayer (quantum well layer and barrier layer), as long as thecompositional wavelength is 680 nm or longer.

Further, according the surface-emission laser diode of the fourththrough seventh aspects of the present invention, it becomes possible toreduce the adversary effects to the device characteristics of thesemiconductor layer, such as decrease of bandgap caused by formation ofnatural super lattices or deterioration of surface morphology orformation of non-optical recombination centers caused by hillock(hill-like defect), by using the (100) GaAs substrate having a surfaceorientation inclined in the direction of a (111)A surface with an anglein the range from 5° to 20°. Further, it becomes possible to conductpolarization control by utilizing the nature of the anisotropicsubstrate. Thus, with the surface-emission laser diode according to thepresent invention, it is not possible to attain the polarization controlas in the case of using the (311)B substrate, which is currently thoughtmost promising, and the anisotropy of optical gain attained with the useof inclined substrate becomes inevitably small, the surface-emissionlaser diode according to the present invention can successfullycompensate for the foregoing decrease of anisotropy of optical gain byproviding a compressive strain to the quantum well active layer andinduce increase of anisotropy of the optical gain. Thereby, it becomespossible to improve the controllability of the polarization directionwith such a surface-emission laser diode.

Further, according to the surface-emission laser diode of the firstthrough seventh aspects, it becomes possible to improve thecontrollability of the polarization direction, by providing anisotropyto the peripheral shape of the active layer as viewed from the opticalexit direction of the surface-emission laser diode such that there isformed a shape elongated in the (111)A direction, such that the effectof increase of the optical gain in the inclined direction of thesubstrate ((111)A surface direction) is added.

Further, by forming the surface-emission layer diode according to thefirst through seventh aspects on the same substrate in plural numbers,it is possible to construct a surface-emission laser diode array. Thus,according to the present invention, precision and controllability ofmesa formation are improved, and it becomes possible to produce thesurface-emission laser diode array having uniform laser characteristicsand good processing reproducibility, with high yield and low cost.

Particularly, with the use of the surface-emission laser diode accordingto the first aspect of the present invention, thermal interferencebetween the elements in the array is suppressed because of theimprovement of the heat dissipation characteristics, and it becomespossible to form a high-density array in which the surface-emissionlaser diode elements are disposed with closer distance from each other.

Further, by applying the construction of integrating a large number ofsurface-emission laser diodes of the first through seventh aspects ofthe present invention, capable of performing high output poweroperation, on the same substrate for the image writing optical system ofan electron photographic image forming apparatus, it becomes possible toachieve high-speed writing by using plural beams at the same time, andthe writing speed is improved significantly. Thereby, it becomespossible to carry out printing without decreasing the speed even in thecase the density of the writing dots is increased. Further, whencompared at the same dot density, the image forming apparatus that usessuch a surface-emission laser diode enables printing at higher speed ascompared with the conventional image forming apparatuses. Further, whenthe surface-emission laser diode of the present invention is applied tocommunication, the data transmission is made by a large number of beamssimultaneously, and high-speed communication is realized.

Further, the surface-emission laser diode operates at low powerconsumption, and thus, it becomes possible, when operated in the stateof being incorporated into an apparatus, to reduce the temperature risein the apparatus.

Further, by using the high output power surface-emission laser diodeaccording to the first through seventh aspects of the present inventionor the surface-emission laser diode array of these for the writingoptical source, it becomes possible to improve the printing speed ascompared with the image forming apparatus that uses a conventionalsurface-emission laser diode.

Alternatively, in the case of printing at the conventional speed, itbecomes possible to reduce the number of the laser arrays, and the yieldof production of the surface-emission laser diode array chip is improvedsignificantly. Further, it becomes possible to reduce the const of theimage forming apparatus. In the case the surface-emission laser diodecapable of controlling the polarization plane is used, reliability ofimage formation is improved. Further, in the case a surface-emissionlaser diode free from Al in the active layer structural part formed of aquantum well active layer and a spacer layer is used, lifetimecomparable to the surface-emission laser diode for telecommunicationpurposes such as the surface-emission laser diode of the 850 nm band isattained, and thus, it becomes possible to reuse the optical writingoptical unit. Thereby, the load to the environment is reduced.

Further, by using the surface-emission laser diode according to thefirst through seventh aspects of the present invention or thesurface-emission laser diode array that uses such surface-emission laserdiodes for the optical source of optical pickup, it becomes possible torealize a handy type optical pickup system of long battery life. Inconventional compact disk devices, a semiconductor layer of 780 nmwavelength is used for the optical writing and playback of recordingmedium, wherein a surface-emission laser diode has power consumptionsmaller than that of an edge-emission laser diode by a factor of 1/10.

Further, by using the surface-emission laser diode of the first throughseventh aspects or the surface-emission laser diode array of suchsurface-emission laser diodes for the high-power optical source ofoptical transmission module or optical transceiver module, it becomespossible to construct an economical high-speed optical transmissionsystem that uses a low cost POF (plastic optical fiber).

With the optical transmission that uses an acrylic POF, asurface-emission laser diode of the oscillation wavelength of 650 nm hasbeen used conventionally for the optical source in view of theabsorption loss characteristics of the optical fiber, while the use ofthe surface-emission laser diode in practical purposes is not beensuccessful. Because of this, LEDs are used currently, while an LED isdifficult to perform high-speed modulation, and it is indispensable toprovide a laser diode in order to realize high-speed transmissionexceeding 1 Gbps.

With the surface-emission laser diode of the present invention havingthe wavelength of 680 nm or longer, a large gain is attained for theactive layer and it is possible to provide a large output. Further, thesurface-emission laser diode of the present invention has excellent hightemperature characteristics. Thus, by using such a surface-emissionlaser diode, it is possible to achieve, in spite of the fact there is anincrease of absorption loss by the fiber, optical transmission of shortrange. Thus, an economical high-speed optical transmission module or anoptical transceiver module that combines a low cost POF with a low costoptical source of surface-emission laser diode is realized. Further, anoptical communication system that uses these is realized. Because suchan optical communicating system is extremely economical, it is suitedfor the optical communication systems of home use, or for use in officerooms, or for the use inside an apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a construction example of a generalsurface-emission laser diode.

FIG. 2 is a diagram showing the fundamental construction example of asurface-emission laser diode of a first mode.

FIG. 3 is a diagram showing the construction of a surface-emission laserdiode of Example 1.

FIG. 4 is a diagram showing the time-change of atomic emission intensityof In (451 nm)/Al (396 nm) ratio of Example 1.

FIG. 5 is a diagram showing the construction of a surface-emission laserdiode of Example 5.

FIG. 6 is a diagram showing the surface-emission laser diode of Example6.

FIG. 7 is a drawing showing the surface-emission laser diode of Example6.

FIG. 8 is a top view diagram showing the surface-emission laser diode ofExample 7.

FIG. 9 is a diagram showing the surface-emission laser diode of Example8.

FIG. 10 is a diagram showing a surface-emission laser diode array ofExample 9.

FIG. 11 is a diagram showing an optical transmission module according toExample 10.

FIG. 12 is a diagram showing an optical transmission/reception moduleaccording to Example 11.

FIG. 13 is a diagram showing a laser printer of according to Example 12.

FIG. 14 is a diagram (top view) showing the outline construction of asurface-emission laser diode array chip (16 beam VCSEL array) used inthe laser printer of FIG. 13.

FIG. 15 is a cross-sectional diagram showing, in principle, a part of afirst construction example according to a twelfth mode of the presentinvention.

FIG. 16 is a cross-sectional diagram showing, in principle, a part of asecond construction example of the twelfth mode of the presentinvention.

FIG. 17 is a cross-sectional diagram showing, in principle, aconstruction example according to a thirteenth mode of the presentinvention.

FIG. 18 is a cross-sectional diagram extracting and showing a structurein the vicinity of the active layer of FIG. 17 with enlarged scale.

FIG. 19 is a plan view diagram of FIG. 17.

FIG. 20 is a cross-sectional view extracting and showing a structure inthe vicinity of the active layer according to a fourteenth mode of thepresent invention with enlarged scale.

FIG. 21 is a diagram showing the construction example of the peripheralpart of the active layer of the surface-emission laser diode for thecase an AlGaInP layer is provided in a cavity region sandwiched by theupper and lower reflectors in a seventeenth and eighteenth modes.

FIG. 22 is a diagram showing a construction example of thesurface-emission laser diode according to a twenty-first mode of thepresent invention.

BEST MODE FOR IMPLEMENTING THE INVENTION

Hereinafter, various modes of the present invention will be explainedwith reference to the drawings.

(First Mode)

According to a first mode of the present invention, there is provided asurface-emission laser diode, comprising:

a semiconductor substrate;

a cavity region formed over said semiconductor substrate, said cavityregion comprising an active layer structural part including at least onequantum well active layer producing a laser light and barrier layers,and a spacer layer provided in a vicinity of said active layerstructural part, said spacer layer comprising at least one material; and

an upper reflector and a lower reflector provided over saidsemiconductor substrate at a top part and a bottom part of said cavityregion,

said cavity region, said upper reflector and said lower reflectorforming a mesa structure over said semiconductor substrate,

said upper reflector and said lower reflector constituting asemiconductor distributed Bragg reflector having a periodic change ofrefractive index and reflecting an incident light by interference ofoptical waves,

at least a part of said semiconductor distributed Bragg reflector beingformed of a layer of small refractive index of Al_(x)Ga_(1-x)As (0<x≦1)and a layer of large refractive index of Al_(y)Ga_(1-y)As (0≦y<x≦1),

said lower reflector being formed of a first lower reflector having alow-refractive index layer of AlAs and a second lower reflector formedon said first lower reflector, said second lower reflector having alow-refractive index layer of AlGaAs,

wherein any one layer constituting said cavity region contains In.

With this first mode of the present invention, any one layerconstituting the cavity region contains In, and the lower semiconductorDBR contains an AlAs/(Al)GaAs-DBR having excellent heat dissipationcharacteristic. Thus, the precision and reproducibility of mesaformation is improved, and it becomes possible to provide theAlAs/(Al)GaAs-DBR to the region close to the cavity region.

With this, temperature rise at the time of driving is suppressed, and ahigh output power surface-emission laser diode having excellenttemperature characteristics is provided. At the same time, asurface-emission laser diode having highly uniform lasercharacteristics, excellent reproducibility for processing and excellentyield is provided.

With the surface-emission laser diode of the first mode, it is possibleto introduce In to at least the lower spacer layer and the upper spacerlayer among the layers constituting the cavity region.

According to such a construction in which In is contained in the spacerlayer having much larger thickness than the active layer structural partamong the layers constituting the cavity region, it becomes possible toform the mesa structure with further improved reproducibility andfurther improved precision, and with this, it becomes possible to obtaina surface-emission laser diode of further improved temperaturecharacteristics, higher output power and further improved uniformity inlaser characteristics, with higher reproducibility of processing andwith higher yield.

Further, it is preferable that the second lower reflector of thesurface-emission laser diode according to the mode of the presentinvention includes 10 pairs or less.

With such a construction, the thickness of the second lowersemiconductor DBR is set to be larger than the precision of the mesaetching and at the same time minimum. With this, it becomes possible toproduce the surface-emission laser diode with improved yield.

Further, the surface emission laser diode of the first mode of thepresent invention can be fabricated by: forming, on a semiconductorsubstrate, a cavity region comprising an active layer structural partincluding at least one quantum well active layer producing a laser lightand a barrier layer; and forming a spacer layer of at least one materialso as to be provided in a vicinity of said active layer structural partto form of a stacked film, such that, in the stacked film, an upperreflector and a lower reflector are provided at a top part and a bottompart of said cavity region, and processing the stacked film by a dryetching process to form a mesa structure. Thereby, with the presentmode, In is incorporated to any one layer constituting said cavityregion, and the height of said mesa structure is controlled bymonitoring light emission of In in the dry etching step.

According to the fabrication process of the foregoing mode, the heightof the mesa structure is controlled in the step of forming the mesastructure by the dry etching of the stacked film, by monitoring thelight emission of In. Because the dry etching is conducted whilemonitoring the light emission of In from the cavity layer, it ispossible to detect the cavity layer positively, and with this, itbecomes possible to form the mesa structure with high reproducibilityand high precision. As a result, it becomes possible with the presentmode to produce the first surface-emission laser diode with highreproducibility and yield.

FIG. 2 shows the fundamental construction of a surface-emission laserdiode 40 according to the first mode of the present invention.

Referring to FIG. 2, the surface-emission laser diode 40 has a VCSELstacked structure formed of consecutive lamination of: a first lowersemiconductor DBR 42 of AlAs/Al_(1-u)Ga_(u)As (0<u≦1) formed on amonocrystalline semiconductor substrate 41 of GaAs, InP, GaP, GaNAs, Si,Ge, or the like, directly or via an intermediate layer by way of anMOCVD process or an MBE process; a second lower DBR 43 ofAl_(1-v)Ga_(v)As/Al_(1-w)Ga_(w)As (0<v<1, 0<w≦1, v<w); a cavity layer 44of a lower spacer layer 44A, an active layer 44B, and an upper spacerlayer 44C, the cavity layer 44 containing In in any one layer of thelower spacer layer 44A, the active layer 44B and the upper spacer layer44C; a selective oxidation layer 45 of Al_(1-t)Ga_(t)As (0≦t≦0.05); andan Al_(1-v)Ga_(v)As/Al_(1-w)Ga_(w)As (0<v<1, 0<w≦1, v<w) upper DBR 46.

Here, it should be noted that the layer containing In is formed of acompound semiconductor material represented asAl_(1-x-y)Ga_(x)In_(y)As_(1-z)P_(z) (0≦x≦1, 0<y≦1, 0<(x+y)≦1, 0≦z≦1),and typically formed of GaInP, GaInAsP, GaInAs, AlGaInAs, AlGaInAsP,wherein the layer may further contain other group III or group V elementsuch as B, N, Sb, Tl, or the like.

Table I represents examples of such a VCSEL stacked structure.

TABLE I Wavelength Quantum Well Band Active Layer Upper/lower (nm)(well/barrier) Spacer Layer Ex 1 650 InGaP/AlGaInP(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P Ex 2 780 GaInAsP/AlGaInP(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P Ex 3 850 GaAs/InGaAsPIn_(0.27)Ga_(0.73)As_(0.44)P_(0.56) Ex 4 980 GaInAs/GaAs GaIn(As)P Ex 51300 GaInNAs/GaAs GaIn(As)P Upper Semiconductor DBR Ex 1Al_(0.95)Ga_(0.05)As/Al_(0.5)Ga_(0.5)As Ex 2Al_(0.95)Ga_(0.05)As/Al_(0.3)Ga_(0.7)As Ex 3Al_(0.95)Ga_(0.05)As/Al_(0.15)Ga_(0.85)As Ex 4 Al_(0.95)Ga_(0.05)As/GaAsEx 5 Al_(0.95)Ga_(0.05)As/GaAs Wavelength Band First Lower Second Lower(nm) Semiconductor DBR Semiconductor DBR Ex 1 650AlAs/Al_(0.5)Ga_(0.5)As Al_(0.95)Ga_(0.05)As/Al_(0.5)Ga_(0.5)As Ex 2 780AlAs/Al_(0.3)Ga_(0.7)As Al_(0.95)Ga_(0.05)As/Al_(0.3)Ga_(0.7)As Ex 3 850AlAs/Al_(0.15)Ga_(0.85)As Al_(0.95)Ga_(0.05)As/Al_(0.15)Ga_(0.85)As Ex 4980 AlAs/GaAs Al_(0.95)Ga_(0.05)As/GaAs Ex 5 1300 AlAs/GaAsAl_(0.95)Ga_(0.05)As/GaAs

Next, a mesa mask pattern is formed by a photoresist, or the like, andthe stacked structure thus formed is held in a processing vessel of adry etching apparatus. Further, a halogen gas such as Cl₂, BCl₃, SiCl₄,CCl₄, CF₄, or the like, is introduced into the processing vessel, and amesa etching is conducted by d dry etching process that uses plasma suchas reactive ion beam etching (RISE) method, induction coupled plasma(ICP) etching method, reactive ion etching method (RIE), or the like.

During such mesa etching process, plasma atomic emission spectrometry isconducted via an observation window provided to the processing vessel ofthe dry etching apparatus, and the time change of emission intensity ofIn at the wavelength of 451 nm is monitored. With the present invention,the atomic emission of In is detected only when the cavity region isetched, and thus, it becomes possible to stop the etching positivelywhile the etching is in progress inside the second semiconductor DBR 43.

Next, the Al(Ga)As layer for selective oxidation 45 is processed inwater vapor, and a current confinement structure of Al_(x)O_(y) isformed. Next, the space surrounding the mesa structure thus formed isfilled with an insulation film 47 of polyimide or SiO2 except for thepart where electrode is to be provided and the part used for opticaloutput. By forming the insulation film 47, the surface-emission laserelement is planarized.

Further, a p-side electrode 48 and an n-side electrode 49 are formed atrespective locations, and fabrication of the surface-emission laserdiode 40 of FIG. 2 is completed.

With the surface-emission laser diode 40, positive carriers and negativecarriers are injected respectively from the p-side electrode 48 and then-side electrode 49, and there are caused photoemission in the activelayer 44B. With this, it becomes possible to obtain a laser light in thevertical direction to the substrate 41. While FIG. 2 shows theconstruction that provides an optical output in the upward direction ofthe substrate 41, it is also possible to construct such that opticaloutput is obtained in the downward direction of the substrate 41.

Here, it should be noted that all of the lower spacer layer 44A, theactive layer 44B, and the upper spacer layer 44C constitutes the cavity44, and the thickness of the cavity, defined as a sum of these, becomes(N₀+1)×λ/n). In many cases, the cavity thickness is λ/n. Here, N₀ is aninteger of 0 or larger, λ is the oscillation wavelength, and n is therefractive index of the semiconductor constituting the cavity 44.

Further, because the active layer 44B is usually implemented in the formof thin quantum well structure, the majority of the cavity length isoccupied by the upper and lower spacer layers 44A and 44C. Further, thethickness of the semiconductor DBR 43 is (1+2×N₀)×λ/(4×n). In manycases, it is λ/(4×n). Thus, the upper and lower spacer layers 44A and44C have a thickness much larger than any other films in the stackedstructure.

From this, strong photoemission of In is achieved at the time of theetching in the case the upper and lower spacer layers 44A and 44Ccontains In, and it becomes possible to control the endpoint of etchingwith high precision. With this, the reproducibility of the mesa etchingis improved significantly.

Further, it becomes possible to control the variation of the etchingdepth, when the foregoing VSCEL stacked structure is etched on a singlewafer by using the foregoing monitoring method, such that the bottom ofthe mesa structure is located within the range of 2 or 3 pairs of theforegoing second lower semiconductor DBR 43. Thus, even when variationof mesa height within the wafer is taken into consideration, orvariation between the wafers when a large number of wafers are etchedsimultaneously is taken into consideration, it is possible to achievethe necessary etching control by using ordinary etching process,provided that there are only ten repetitions in the maximum for the highrefractive index layer and the low refractive index layer in the secondlower semiconductor DBR 43.

Thus, according to the present mode, it becomes possible to use theconstruction in which the AlAs/(Al)GaAs-DBR 43 of small thermalresistance is provided to the vicinity of the cavity of thesurface-emission laser diode. Thereby, the efficiency of heatdissipation is improved and the temperature rise at the time of drivingis suppressed, and a high output power surface-emission laser diode ofexcellent temperature characteristics is obtained.

In the case of using the material or thickness in which the oxidationrate of the low refractive indeed layer in the first lower semiconductorDBR 42 is larger than the oxidation rate of the selectively oxidizedsemiconductor layer 45, the mesa etching should not reach the firstlower semiconductor DBR 42. Such a situation is not limited to the casein which the selective oxidized layer 45 and the low refractive indexlayer of the first lower semiconductor DBR 42 are both formed of AlAs.For example, there can be a case in which the layer for selectiveoxidation 45 is not AlAs but contains a small amount of Ga and the lowrefractive index layer of the first lower semiconductor DBR is not AlAsbut also contains Ga.

Even in such cases, however, satisfactory heat dissipation effect isattained as long as the low refractive index layer of the second lowersemiconductor DBR 43 is formed of the material having the oxidation ratesmaller than the oxidation rate of the layer for selective oxidation 45and when the low refractive index layer of the first lower semiconductorDBR 42 is of the composition or material in which the thermal resistanceis smaller than the low refractive index layer of the second layersemiconductor DBR 43.

Further, in addition to the method of simply monitoring the photoemission intensity of In, it is also possible, with the emissionspectrometry of In, to monitor the ratio of the emission intensity of Into the emission intensity of other constituent element, or the ratio ofthe emission intensity of In to the emission intensity of the wavelengthdifferent from any of the foregoing, so as to cancel out the effect ofvariation of the plasma state.

In order to control the etching bottom to be right underneath thecavity, it is also possible to use GaInP or AlGaInP for the uppermostlayer of the lower semiconductor DBR 43 and use GaAs or AlGaAs for theupper layer that includes the cavity. With such a case, it becomespossible to conduct selective oxidation by using a H₂SO₄/H₂O₂/H₂Osolution. However, with such a wet etching process, control of the mesawidth is difficult, and there tends to arise the problems in that,because of etching anisotropy, there is formed a mesa of asymmetricshape. In view of the foregoing situations, it is preferable to conductthe mesa etching by a dry etching process.

With the foregoing construction, it should be noted that, while the AlAsselective oxidized layer 45 is provided in the vicinity of the activelayer 44B, the location is not limited, in view of the cases in which itis provided inside the DBR as the low refractive index layer of theupper semiconductor DBR 46 or the low refractive index layer of thelower semiconductor DBRs 42 and 43.

Further, while explanation has been made for the case in which In iscontained only in the cavity region 44, all what is required is that thesituation of the etching of the cavity region 44 is grasped and that theetching does not reach the first lower reflector 42, and thus, it isalso possible to provide a layer containing In other than the cavityregion, such as a part of the reflector 43 closest to the cavity region44.

(Second Mode)

According to a second mode, there is provided a surface emission laserdiode, in which a part of the spacer layers 44A and 44C is formed of(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) in the surface-emissionlaser diode 40 of the first mode and the active layer 44B is formed of aquantum well layer of Ga_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦c≦1) and abarrier layer of Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1).

By using (Al_(a)Ga_(1-a)) (0<a≦1, 0≦b≦1) for a part of the spacer layers44A and 440, it becomes possible to increase the bandgap differencebetween the space layer and the quantum well active layer as comparedwith the case of forming the spacer layer by the AlGaAs system.

Table 2 shows the bandgap difference caused between the spacer layer andthe quantum well layer and between the barrier layer and the quantumwell layer for the typical material compositions used in thesurface-emission layer diode of the 780 nm and 850 nm wavelengths thatuses the AlGaAs (spacer)/AlGaAs (quantum well active layer) system andfurther in the surface-emission laser diode of the 780 nm wavelengthband that uses the AlGaInP (spacer) layer)/GaInPAs (quantum well activelayer).

TABLE 2 Wavelength 780 nm Spacer layer/quantum well active layerAlGaAs/AlGaAs system material Spacer layer Al_(0.6)Ga_(0.4)As (Eg =2.0226 eV) Active Quantum well Al_(0.12)Ga_(0.88)As layer active layer(Eg = 1.5567 eV) Barrier Al_(0.3)Ga_(0.7)As layer (Eg = 1.7755 eV) Egdifference 465.9 meV (ΔEg) between spacer layer and quantum well layerEg difference 218.8 meV (ΔEg) between barrier layer and quantum welllayer Wavelength 780 nm AlGaInP/GaInPAs system material Spacer layer(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P (Eg(x = 0.7) = 2.289 eV) Active Quantumwell GaInPAs (compressive strain) layer active layer (Eg = 1.5567 eV)Barrier Ga_(x)In_(1−x)P (tensile strain) layer (Eg(x = 0.6) = 2.02 eV)Eg difference 743.3 meV (ΔEg) between spacer layer and quantum welllayer Eg difference 463.3 meV (ΔEg) between barrier layer and quantumwell layer Wavelength 850 nm (Ref.) AlGaAs/GaAs system material Spacerlayer Al_(0.6)Ga_(0.4)As (Eg = 2.0226 eV) Active Quantum well GaAs layeractive layer (Eg = 1.42 eV) Barrier Al_(0.3)Ga_(0.7)As layer (Eg =1.7755 eV) Eg difference 602.6 meV (ΔEg) between spacer layer andquantum well layer Eg difference 355.5 meV (ΔEg) between barrier layerand quantum well layer

As can be seen in Table 2, it becomes possible to secure a large bandgapdifference with the surface-emission laser diode of the 780 nm band thatuses the AlGaInP (spacer layer)/GaInPAs (quantum well active layer)system as compared with the surface-emission laser diode of the 780 nmwavelength band of the AlGaAs/AlGaAs system or the surface-emissionlaser diode of the 850 nm band of the AlGaAs/AlGaAs system.

Further, with the surface-emission laser diode of such a construction,it is possible to use a compressive strain composition for the quantumwell active layer. With increase of strain, band splitting between theheavy holes and light holes is increased, while this leads to increaseof the gain and decrease of threshold value of laser oscillation.Thereby, the efficiency of laser oscillation is improved and the laseroutput is increased. It should be noted that this effect is not achievedwith the surface-emission laser diode of the 850 nm band that uses theAlGaAs/AlGaAs system.

Thus, according to the surface-emission laser diode of the presentinvention that uses the AlGaInP/GaInPAs material system, the thresholdvalue is decreased as compared with the surface-emission laser diode of850 nm that uses the AlGaAs/AlGaAs system, and it becomes possible torealize a high output power laser of improved laser oscillationefficiency.

Further, according to the surface-emission laser diode of the secondmode, the efficiency of carrier confinement is improved and thethreshold value is decreased also by the increase of gain as a result ofuse of the strained quantum well active layer, and as a result, itbecomes possible to reduce the reflectivity of the exit-side DBR. Withthis, it is possible to attain further increase of the optical output.

Further, with the surface-emission laser diode of the second mode, thequantum well active layer is formed of Ga_(c)In_(1-c)P_(d)As_(1-d)(0≦c≦1, 0≦d≦1) and the barrier layer is formed ofGa_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1), while it should be notedthat this material is free from Al, and thus, incorporation of oxygeninto the quantum well active layer is reduced because of the fact thatthe active layer structural part 44, formed of the quantum well activelayer and the layer adjacent thereto, is free from Al. Thereby,formation of non-optical recombination center can be suppressed and itbecomes possible to realize a surface-emission laser diode of longlifetime.

Thus, by using AlGaInP material for a part of the spacer layers 44A and44C and by using GaInPAs for the barrier layer or quantum well activelayer, the optical gain of the active layer is increased, and it becomespossible to realize a reliable high output power surface-emission laserdiode that has a low threshold value and oscillates at the wavelengthband shorter than 850 nm.

In order to attain the foregoing effect, it is preferable to considerthe surface orientation of the substrate 41 as explained below.

In the surface-emission laser diode that uses AlGaInP or GaInP for theactive region, in particular, it is preferable to use a (100) GaAssubstrate having a surface orientation inclined in the direction of a(111)A surface by an angle (inclination angle) of the range of 5° to 20°for the substrate 41. The reason of this is that, when the surfaceorientation of the substrate is close to (100), there is caused problemssuch as decrease of bandgap by formation of natural superlattices,deterioration of surface morphology caused by hillock (hill-likedefect), formation of non-optical recombination centers, and the like,and there is a possibility that the device characteristics of the laserdiode formed on the substrate is adversary affected.

When the surface orientation of the substrate is inclined from (100)surface to the direction of (111)A surface, formation of the naturalsuperlattice is suppressed in correspondence to the inclination angle.Thus, the bandgap changes sharply in the range of the inclination angleof 10° to 15°, and thereafter, the band gap approaches the nominalbandgap (bandgap value of the mixed crystal). Further, formation ofhillocks is gradually suppressed.

On the other hand, when the inclination angle in the direction of (111)Asurface exceeds 20°, crystal growth on the substrate becomes difficult.Thus, with the red color laser (630 nm to 680 nm) that uses the AlGaInPmaterial, the substrate inclined to the angle in the range of 5° to 20°(in many cases in the range of 7° to 15°) is used commonly. This appliesnot only to the case AlGaInP is used for the spacer layer (claddinglayer), but also to the case in which GaInP is used for the barrierlayer as in the example of Table 2. Further, in anticipation ofadversary effect caused also in the case the barrier layer and thequantum well active layer are formed of GaInPAs, it is preferable to usea (100) GaAs substrate having a surface orientation inclined in thedirection of (111)A surface with the angle in the range of 5° to 20°(more preferably with the angle in the range of 7° to 15°), for thegrowth of these materials.

On the other hand, in the case the surface orientation of the substrate41 is inclined in the direction of (111)A surface, it is not possible touse the control technology of polarization angle (polarizationdirection) that uses the optical gain anisotropy of (311)B substrate andthought as being most promising technology at the current juncture.Thus, while the present mode can suppress the cost of the substrate byusing an inclination angle (range of 5° to 20°) smaller than the (311)Bsubstrate (inclination angle of 25°) and that cleaving is made easilyand handling of the substrate becomes easier, the optical gainanisotropy that can be obtained becomes inevitably small.

Thus, the present mode compensates for the decrease of the optical gainanisotropy by the increase of the optical gain anisotropy induced byapplying a compressive strain to the quantum well active layer.

It should be noted that, while the foregoing example limits thewavelength to be shorter than 850 nm, this is merely because a verylarge advantage is obtained with this wavelength range, and similareffect is obtained also in the wavelength longer than 850 nm.

Thus, with the present mode, a surface-emission laser diode of highoutput power and excellent reliability is obtained such that the surfacelaser diode operates at the wavelength of 850 nm or shorter and has thefeature of, in addition to the features of the first mode of large gainfor the active layer and reduced threshold value, controlledpolarization direction.

(Third Mode)

According to a third mode, there is provided a surface-emission laserdiode, comprising: a (100) GaAs substrate having a surface orientationinclined in a direction of a (111)A surface by an angle of 5° to 20°; acavity region provided over said GaAs substrate, said cavity regionincluding an active layer structural part comprising at least one layerof quantum well active layer producing a laser light and barrier layers,and a spacer layer provided in a vicinity of said active layerstructural part, said spacer layer comprising at least one material; andan upper reflector and a lower reflector provided at a top part and abottom part of said cavity region, said cavity region and said upper andlower reflectors forming a mesa structure over said GaAs substrate, saidupper reflector and said lower reflector comprising a semiconductordistributed Bragg reflector having a periodic change of refractive indexand reflecting an incident light by interference of optical waves, atleast a part of said semiconductor distributed Bragg reflector beingformed of a layer of small refractive index of Al_(x)Ga_(1-x)As (0<x≦1)and a layer of large refractive index of Al_(y)Ga_(1-y)As (0≦y<x≦1), apart of said spacer layer comprising (Al_(a)Ga_(1-a))_(b)In_(1-b)P(0<a≦1, 0≦b≦1), said quantum well active layer comprisingGa_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1) said barrier layerscomprising Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1), said quantum wellactive layer having a compressive strain, said active layer structuralpart having a shape anisotropy elongated in a direction of a (111)Asurface as viewed from a direction of light emission.

When controlling the polarization angle (polarization direction) byusing the optical gain anisotropy caused by inclining the surfaceorientation of the substrate in the direction of (111)A surface with thesurface emission laser diode of the present invention, it is notpossible to utilize the effect of the (311)B substrate, which is thoughtas being most promising at the current juncture, because of the use ofthe inclination angle (range of 5° to 20°), which is smaller than the(311)B substrate (inclination angle of 25°).

Thus, with the third mode of the present invention, this decrease iscompensated for by the increase of the optical gain anisotropy obtainedby applying a compressive strain to the quantum well active layer, andfurther by providing anisotropy to the outer shape of the active layeras viewed from the direction of optical emission of the surface-emissionlaser diode, such that the optical gain in the inclined direction((111)A surface direction) of the substrate is increased by providing anelongated shape in the direction of (111)A surface to the active layer.With this, the optical gain in the direction of the inclination angle(direction of (111)A surface) is increased further and controllabilityof the deflection angle is improved.

(Fourth Mode)

A fourth mode of the present invention provides a surface-emission laserdiode of the second or third mode in which the barrier layer has atensile strain.

In the material of the GaInPAs system used for the barrier layer of thequantum well active layer in the surface-emission laser diode, GaInP hasthe largest bandgap when compared with the same lattice constant.Further, a larger bandgap is obtained with the material having a smallerlattice constant. Thus, by using a material of the GaInP system of smalllattice constant for the barrier layer, it is possible to realize alarge band discontinuity between the barrier layer and the quantum wellactive layer, and it becomes possible to increase the gain of thesurface-emission laser diode. With this, the surface-emission laserdiode can perform high output power operation with low threshold value.For example, it should be noted that the bandgap of the tensile-strainedGa_(0.6)In_(0.4)P layer is 2.02 eV, the bandgap of the Ga_(0.5)In_(0.5)Plattice matched layer is 1.87 eV, and thus the Ga_(0.6)In_(0.4)P tensilestrain layer has a bandgap larger by 150 meV.

(Fifth Mode)

With the fifth mode of the present invention, there is provided asurface-emission laser diode of any of the second through fourth modesin which the oscillation wavelength is about 680 nm or longer.

Comparing the surface-emission laser diode of the present mode with thesurface-emission laser diode of 780 nm that uses the active layer of theAlGaAs/AlGaAs system, it can be seen that the bandgap difference betweenthe Al_(x)Ga_(1-x)As (x=0.6, Eg=2.0226 eV), which provides the largestbandgap in the typical compositional range of the Al_(x)Ga_(1-x)As(0<x≦1) system spacer layer used with the surface-emission laser diodeof the AlGaAs/AlGaAs system, and the active layer of the compositionalwavelength of 780 nm (Eg=1.5567 eV) is generally equal to the bandgapdifference (460 meV) between (Al_(a)Ga_(1-a))_(b)In_(1-b)P (a=0.7,b=0.5, Eg=2.289 eV), which provides the largest bandgap in the typicalcompositional range of the (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1)spacer layer used with the surface emission laser diode of the presentmode and the active layer of the compositional wavelength of 680 nm(Eg=1.8233 eV).

Further, with regard to the bandgap difference between the barrier layerand the quantum well active layer, it should be noted that the bandgapdifference to the active layer of the compositional wavelength of 680 nmbecomes about 200 meV, assuming that the barrier layer has thecomposition Ga_(e)In_(1-e)P_(f)As_(1-f) (e=0.6, f=1, Eg=2.02 eV), whilethis is generally equal to the case of the 780 nm surface-emission laserdiode of the active layer of the AlGaAs/AlGaAs system.

This means, that by using the spacer layer of the AlGaInP system, itbecomes possible to achieve the carrier confinement equivalent or largerthan the surface-emission laser diode of 780 nm that uses theAlGaAs/AlGaAs system active layer, even in the case the surface-emissionlaser diode is the one that uses the Al-free active layer (quantum wellactive layer and the barrier layer), as long as the compositionalwavelength is longer than 680 nm. In practice, the characteristicsequivalent or more are attained in view of the effect of the strainedquantum well active layer.

(Sixth Mode)

According to the sixth mode, there is provided a surface-emission laserdiode array in which plural surface-emission laser diodes of any of thefirst through fifth modes are constructed on a common substrate.

Generally, a surface-emission laser diode can easily construct a laserarray in view of its surface-emission construction. Further, becauseeach surface-emission laser diode element is formed with an ordinarysemiconductor process, it is possible to form the individualsurface-emission laser diode elements with high positional precision.Particularly, according to the present invention, controllability ofetching at the time of mesa formation is improved, and as a result,there is attained an improvement of yield, and it becomes possible toreduce the production cost.

Further, according to the present invention, heat dissipation efficiencyof the lower DBR is improved, and as a result, thermal interferencebetween the elements in the array is reduced, and it becomes possible toincrease the output of the individual devices and increase the formationdensity of the elements.

Further, by using the laser array in which a large number of high outputpower surface-emission laser diodes of the present invention, eachhaving a polarization direction controlled in a predetermined direction,are integrated on a common substrate, it becomes possible to realizesimultaneous writing by plural beams in a writing optical system ofimage forming apparatus, or the like, and the writing speed is improvedsignificantly. Further, according to such a construction, it becomespossible to conduct printing without decreasing the printing speed evenin the case the writing dot density is increased. In the same context,it is possible to increase the printing speed when the writing dotdensity is held constant. When such a laser array is applied tocommunication, it becomes possible to carry out simultaneous datatransmission with plural beams, and high speed communicating becomespossible. Further, because the surface-emission laser diode operateswith low power consumption, it becomes possible to suppress thetemperature rise inside an apparatus when the laser diode is assembledinto the apparatus.

(Seventh Mode)

According to a seventh mode of the present invention, there is providedan image forming apparatus that uses the surface-emission laser diode ofany of the first through fifth modes or the surface-emission laser diodearray of the sixth mode for the writing optical source.

Because the surface-emission laser diode or the surface-emission laserdiode array of the present invention has a controlled polarizationdirection and operates with high output power, the image formingapparatus of the present embodiment is capable of performing high speedprinting as compared with the image forming apparatus that uses aconventional surface-emission laser diode array. In the case the imageforming apparatus is designed to provide a conventional printing speed,on the other hand, it becomes possible to reduce the number of the laserarrays used. Thereby, the yield of production of the surface-emissionlaser diode array chip is improved significantly, and the productioncost of the image forming apparatus can be reduced. Further, because thesurface-emission laser diode of the present embodiment uses an Al-freeactive layer for the active layer, it becomes possible to attain thelifetime equivalent to the lifetime (estimated of 100 million hours atroom temperature is reported) the communication purpose surface-emissionlaser diode such as the surface-emission laser diode of the 850 nm band,and thus, it becomes possible to reuse the optical unit for opticalwriting. With this, the load to the environment can be reduced.

(Eighth Mode)

According to an eighth mode of the present invention, there is providedan optical pickup system that uses the surface-emission laser diodeaccording to any of the first through fifth modes or thesurface-emission laser diode array of the sixth mode as the opticalsource.

Conventionally, a compact disk apparatus uses the wavelength of 780 nmfor the writing and playback optical source to an optical media. Becausethe surface-emission laser diode has a power consumption smaller thanthat of an edge-emission laser diode by the factor of ten, it becomespossible to realize a handy type optical pickup system characterized bylong battery life, by using the surface-emission laser diode of the 780nm band for the playback optical source.

(Ninth Mode)

According to a ninth mode, there is provided an optical transmissionmodule that uses the surface-emission laser diode of any of the firstthrough fifth modes or the surface-emission laser diode array of thesixth mode.

With the optical transmission that uses an acrylic POF, asurface-emission laser diode of the oscillation wavelength of 650 nm hasbeen used conventionally for the optical source in view of theabsorption loss characteristics of the optical fiber, while the use ofthe surface-emission laser diode in practical purposes is not beensuccessful. Because of this, LEDs are used currently, while an LED isdifficult to perform high-speed modulation, and it is indispensable toprovide a laser diode in order to realize high-speed transmissionexceeding 1 Gbps.

With the surface-emission laser diode of the present invention havingthe wavelength of 680 nm or longer, a large gain is attained for theactive layer and it is possible to provide a large output. Further, thesurface-emission laser diode of the present invention has excellent hightemperature characteristics. Thus, by using such a surface-emissionlaser diode, it is possible to achieve, in spite of the fact there is anincrease of absorption loss by the fiber, optical transmission of shortrange. Thus, an economical high-speed optical transmission module thatcombines a low cost POF with a low cost optical source ofsurface-emission laser diode is realized.

(Tenth Mode)

According to a tenth mode of the present invention, there is provided anoptical transceiver module that uses the surface-emission laser diode ofany of the first through fifth modes or the surface-emission laser diodearray of the sixth mode as an optical source.

With the optical transmission that uses an acrylic POF, asurface-emission laser diode of the oscillation wavelength of 650 nm hasbeen used conventionally for the optical source in view of theabsorption loss characteristics of the optical fiber, while the use ofthe surface-emission laser diode in practical purposes is not beensuccessful. Because of this, LEDs are used currently, while an LED isdifficult to perform high-speed modulation, and it is indispensable toprovide a laser diode in order to realize high-speed transmissionexceeding 1 Gbps.

With the surface-emission laser diode of the present invention havingthe wavelength of 680 nm or longer, a large gain is attained for theactive layer and it is possible to provide a large output.

Further, the surface-emission laser diode of the present invention hasexcellent high temperature characteristics. Thus, by using such asurface-emission laser diode, it is possible to achieve, in spite of thefact there is an increase of absorption loss by the fiber, opticaltransmission of short range. Thus, an economical high-speed opticaltransceiver module that combines a low cost POF with a low cost opticalsource of surface-emission laser diode is realized.

(Eleventh Mode)

According to an optical communication system of the eleventh mode of thepresent invention, there is provided an optical communication systemthat uses the surface-emission laser diode according to any of the firstthrough fifth modes or the surface-emission laser diode array of thesixth mode as the optical source.

With the optical transmission that uses an acrylic POF, asurface-emission laser diode of the oscillation wavelength of 650 nm hasbeen used conventionally for the optical source in view of theabsorption loss characteristics of the optical fiber, while the use ofthe surface-emission laser diode in practical purposes is not beensuccessful. Because of this, LEDs are used currently, while an LED isdifficult to perform high-speed modulation, and it is indispensable toprovide a laser diode in order to realize high-speed transmissionexceeding 1 Gbps.

With the surface-emission laser diode of the present invention havingthe wavelength of 680 nm or longer, a large gain is attained for theactive layer and it is possible to provide a large output. Further, thesurface-emission laser diode of the present invention has excellent hightemperature characteristics. Thus, by using such a surface-emissionlaser diode, it is possible to achieve, in spite of the fact there is anincrease of absorption loss by the fiber, optical transmission of shortrange. Thus, an economical high-speed optical transceiver module thatcombines a low cost POF with a low cost optical source ofsurface-emission laser diode is realized.

Because such a system is extremely economical, it is suited forproviding an optical communication system used in common homes and inoffice rooms or in an apparatus.

Hereinafter, examples of the present invention will be explained.

Example 1

FIG. 3 shows the construction of a surface-emission laser diode 60according to Example 1 of the present invention. It should be noted thatthe surface-emission laser diodes of Examples 2, 3 and 4 to be describedlater have also the construction similar to that of FIG. 3.

Referring to FIG. 3, Example 1 forms a VCSEL stacked structure on ann-GaAs monocrystal (100) substrate 61 by consecutively stacking, by wayof an MOCVD process, a first lower semiconductor DBR 62 in which ann-AlAs/Al_(0.3)Ga_(0.7)As pair is repeated for 42.5 times, a secondlower semiconductor DBR 63 in which ann-Al_(0.95)Ga_(0.05)As/Al_(0.3)Ga_(0.7)As pair is repeated six times, aGa_(0.5)In_(0.5)P lower spacer layer 64A, a GaInAsP/Ga_(0.5)In_(0.5)P(well/barrier) TQW active layer 64B, a Ga_(0.5)In_(0.5)P upper spacerlayer 64C, a p-AlAs selective oxidation layer 65, an upper semiconductorDBR 66 in which a p-Al_(0.95)Ga_(0.05)As/Al_(0.3)Ga_(0.7)As pair isrepeated 34.5 times, and a p-GaAs contact layer 67. Here, the lowerspacer layer 64A, the active layer 64B and the upper spacer layer 64Cforms a cavity structure 64.

Next, a circular mesa mask is patterned on such a VCSEL stackedstructure thus formed by a photoresist, and mesa etching is started by areactive ion beam etching (RIBE) method by introducing a Cl₂ gas.

In the present embodiment, the emission intensity of In (451 nm) and theemission intensity of Al (396 nm) are obtained during the mesa etchingprocess by using a plasma atomic emission spectrometer, and the timechange of the ratio (In/Al ratio) is monitored.

FIG. 4 shows the time change of the In (451 nm)/Al (396 nm) emissionintensity ratio obtained with Example 1.

Referring to FIG. 4, it can be seen that emission of In (451 nm) isdetected after several minutes after the commencement of the etching,while this emission disappears in the meantime. Thus, when the etchingis terminated at the moment the emission of In has disappeared, the mesaetching stops at the third layer from the top of the second lowersemiconductor DBR 63, and the mesa structure M of FIG. 3 is formed.

Next, at the moment the mesa structure M is formed, the AlAs selectiveoxidation layer 65 is annealed in water vapor ambient at 400° C., andthere is formed a current confinement structure inside the AlAs layerfor selective oxidation 65 such that the non-oxidized AlAs region has anarea of 25 μm². Further, the surrounding area of the mesa structure isfilled with a polyimide protective film 68 excluding the part used forelectrode contact and the part used for optical exit.

Next, a p-side electrode film 69 is deposited on the top surface of themesa structure M in contact with the p-type contact layer 67 by anevaporation deposition process, and an opening for optical output isformed by a liftoff process. Further, an n-side electrode 70 is providedon the rear side of the substrate 61 and the surface-emission laserdiode of the construction shown in FIG. 3 is fabricated.

With the surface-emission laser diode 60 of Example 1, positive carriersand negative carriers are injected respectively from the p-sideelectrode 69 and the n-side electrode 70, and the laser beam of thewavelength of 780 nm is emitted in the direction perpendicular to thesubstrate 61 via the opening formed in the upper electrode 69.

With the surface-emission laser diode 60 of Example 1, the VCSEL stackedstructure containing In in the entirety of the cavity structure 64 isetched while monitoring the photoemission of In, and it becomes possibleto detect the cavity structure 64 conveniently at the time of etching.Thus, it becomes possible to form the second lower semiconductor DBR 63with the number of layers smaller than 4/7 of the total lowersemiconductor DBR. As a result, temperature rise of the device issuppressed and it becomes possible to drive the surface-emission laserdiode with higher output power. Further, with the present embodiment,mesa etching is conducted with good reproducibility, and the mesaheights become uniform, and it becomes possible to obtainsurface-emission laser diodes of uniform laser characteristics with highyield.

Example 2

Next, a surface-emission laser diode 80 according to Example 2 of thepresent invention will be explained. As noted before, thesurface-emission laser diode 80 of Example 2 has a construction similarto that of FIG. 3.

Referring to FIG. 3, Example 2 forms a VCSEL stacked structure on ann-GaAs monocrystal (100) substrate 61 by consecutively stacking, by wayof an MOCVD process, a first lower semiconductor DBR 62 in which ann-AlAs/Al_(0.5)Ga_(0.5)As pair is repeated for 47.5 times, a secondlower semiconductor DBR 63 in which ann-Al_(0.95)Ga_(0.05)As/Al_(0.5)Ga_(0.5)As pair is repeated ten times, a(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P lower spacer layer 64A, anIn_(0.46)Ga_(0.54)P/(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P (well/barrier) TQWactive layer 64B, a (Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P upper spacer layer64C, a p-AlAs selective oxidation layer 65, an upper semiconductor DBR66 in which a p-Al_(0.95)Ga_(0.05)As/Al_(0.5)Ga_(0.5)As pair is repeated40.5 times, and a p-GaAs contact layer 67.

Next, a circular mesa mask is patterned on such a VCSEL stackedstructure thus formed by a photoresist, and mesa etching is conducted byan ICP etching method by introducing a Cl₂ gas.

In the present embodiment, time-change of the emission intensity of In(451 nm) is monitored by a plasma emission spectrometer during thisplasma etching process. With progress of the etching, emission of In(451 nm) is detected, while this emission disappears in the meantime.Thus, by terminating the etching process at the moment the emission ofIn has disappeared, a mesa structure M, in which the mesa etching hasstopped in the second lower semiconductor DBR 63, is obtained.

Next, at the moment the mesa structure M is formed, the AlAs selectiveoxidation layer 65 is annealed in water vapor ambient at 400° C., andthere is formed a current confinement structure inside the AlAs layerfor selective oxidation 65 such that the non-oxidized AlAs region has anarea of 25 μm². Further, the surrounding area of the mesa structure isfilled with a polyimide protective film 68 excluding the part used forelectrode contact and the part used for optical exit.

Next, a p-side electrode film 69 is deposited on the top surface of themesa structure M in contact with the p-type contact layer 67 by anevaporation deposition process, and an opening for optical output isformed by a liftoff process. Further, an n-side electrode 70 is providedon the rear side of the substrate 61 and the surface-emission laserdiode of the construction shown in FIG. 3 is fabricated.

With the surface-emission laser diode 80 of Example 2, positive carriersand negative carriers are injected respectively from the p-sideelectrode 69 and the n-side electrode 70, and the laser beam of thewavelength of 650 nm is emitted in the direction perpendicular to thesubstrate 61 via the opening formed in the upper electrode 69.

With the surface-emission laser diode 80 of Example 2, it becomespossible to obtain surface-emission laser diodes of uniform lasercharacteristics similar to the surface-emission laser diode of Example 1with high yield.

Example 3

Next, a surface-emission laser diode 100 according to Example 3 of thepresent invention will be explained. As noted before, thesurface-emission laser diode 100 of Example 3 has a construction similarto that of FIG. 3.

Referring to FIG. 3, Example 3 forms a VCSEL stacked structure on ann-GaAs monocrystal (100) substrate 61 by consecutively stacking, by wayof an MOCVD process, a first lower semiconductor DBR 62 in which ann-AlAs/Al_(0.5)Ga_(0.5)As pair is repeated for 40.5 times, a secondlower semiconductor DBR 63 in which ann-Al_(0.95)Ga_(0.05)As/Al_(0.15)Ga_(0.85)As pair is repeated five times,an In_(0.27)Ga_(0.73)As_(0.44)P_(0.56) lower spacer layer 64A, anGaAs/In_(0.27)Ga_(0.73)As_(0.44)P_(0.56) (well/barrier) TQW active layer64B, an In_(0.27)Ga_(0.73)As_(0.44)P_(0.56) upper spacer layer 64C, ap-AlAs selective oxidation layer 65, an upper semiconductor DBR 66 inwhich a p-Al_(0.95)Ga_(0.05)/Al_(0.15)Ga_(0.85)As pair is repeated 30.5times, and a p-GaAs contact layer 67.

Next, mesa etching is conducted to the VCSEL stacked structure similarlyto Example 2, and after similar thermal annealing process, thesurrounding area of the mesa structure M is filled with a polyimide film68, followed by electrode formation. With this, the surface-emissionlaser diode 100 of FIG. 3 is obtained.

With the surface-emission laser diode 100 of Example 3, positivecarriers and negative carriers are injected respectively from the p-sideelectrode 69 and the n-side electrode 70, and the laser beam of thewavelength of 850 nm is emitted in the direction perpendicular to thesubstrate 61 via the opening formed in the upper electrode 69.

With the surface-emission laser diode of Example 3, too, it becomespossible to obtain surface-emission laser diodes of good heatdissipation characteristics and uniform laser characteristics similar tothe surface-emission laser diode of Example 1 with high yield.

Example 4

Next, a surface-emission laser diode 120 according to Example 4 of thepresent invention will be explained. As noted before, thesurface-emission laser diode 120 of Example 4 has a construction similarto that of FIG. 3.

Referring to FIG. 3, Example 4 forms a VCSEL stacked structure on ann-GaAs monocrystal (100) substrate 61 by consecutively stacking, by wayof an MOCVD process, a first lower semiconductor DBR 62 in which ann-AlAs/GaAs pair is repeated for 32.5 times, a second lowersemiconductor DBR 63 in which an n-Al_(0.95)Ga_(0.05)As/n-GaAs pair isrepeated three times, a Ga_(0.5)In_(0.5)P lower spacer layer 64A, aGaInNAs/GaAs (well/barrier) TQW active layer 64B, a Ga_(0.5)In_(0.5)Pupper spacer layer 64C, a p-AlAs selective oxidation layer 65, an uppersemiconductor DBR 66 in which a p-Al_(0.95)Ga_(0.05)As/p-GaAs pair isrepeated 26 times, and a p-GaAs contact layer 67.

Next, mesa etching is conducted to the VCSEL stacked structure similarlyto Example 2, and after similar thermal annealing process, thesurrounding area of the mesa structure M is filled with a polyimide film68, followed by electrode formation. With this, the surface-emissionlaser diode 120 of FIG. 3 is obtained.

With the surface-emission laser diode 120 of Example 4, positivecarriers and negative carriers are injected respectively from the p-sideelectrode 69 and the n-side electrode 70, and the laser beam of thewavelength of 1300 nm is emitted in the direction perpendicular to thesubstrate 61 via the opening formed in the upper electrode 69.

With Example 4, the active layer contains GaInNAs, and thus, in additionto the function and effect of Example 1 explained before, it becomespossible to construct a laser device of the 1.3 μm band also on the GaAssubstrate. Thereby, it becomes possible to use the high performance DBRof the AlGaAs system and it becomes further possible to use theselectively oxidized confinement structure. Further, because of largeband discontinuity between the barrier layer or spacer layer and theGaInNAs active layer, the efficiency of carrier confinement is improvedfurther and the characteristic temperature is improved further. Thereby,a surface-emission laser diode applicable widely for the optical sourceof optical transmission is obtained.

Further, because the surface-emission laser diode of the presentembodiment is the device of the 1.3 μm band, it is possible to conductthe mesa etching such that the bottom of the etching is located insidethe second lower DBR 63, even in the case the semiconductor layersconstituting the DBRs 62 and 63 have a large thickness and the secondlower DBR 63 has the thickness of only three pairs. With this, heatdissipation characteristics and temperature characteristics are improvedfurther, and it becomes possible to drive the surface-emission laserdiode with further higher power.

Example 5

FIG. 5 shows the construction of a surface-emission laser diode 140according to Example 5.

Referring to FIG. 5, Example 5 forms a VCSEL stacked structure on anp-GaAs monocrystal (100) substrate 61 by consecutively stacking, by wayof an MOCVD process, a first lower semiconductor DBR 82 in which ap-AlAs/p-Al_(0.15)Ga_(0.85)As pair is repeatedly stacked for 39.5 times,a second lower semiconductor DBR 83 in which ap-Al_(0.95)Ga_(0.05)As/p-Al_(0.15)Ga_(0.5) pair is repeatedly stackedfor six times, a p-AlAs layer for selective oxidation 84, anIn_(0.27)Ga_(0.73)As_(0.44)P_(0.56) lower spacer layer 85A, aGaAs/In_(0.27)Ga_(0.73)P_(0.56) (well/barrier) TQW active layer 85B, anIn_(0.27)Ga_(0.73)As_(0.44)P_(0.56) upper spacer layer 84C, an uppersemiconductor DBR 86 in which ann-Al_(0.95)Ga_(0.05)As/n-Al_(0.15)Ga_(0.85)As pair is repeatedly stackedfor 30.5 times, and an n-GaAs contact layer 87. Here, the lower spacerlayer 85A, the active layer 85B and the upper spacer layer 84 c forms anactive structural part 85 serves for the cavity.

Next, mesa etching is conducted to the VCSEL stacked structure similarlyto Example 2, and after similar thermal annealing process, thesurrounding area of the mesa structure M is filled with a polyimide film80, followed by electrode formation. With this, the surface-emissionlaser diode 120 of FIG. 3 is obtained. With the present invention, itshould be noted that the p-side electrode 89 is provided on the rearsurface of the substrate 81 and the n-side electrode 88 is formed on thecontact layer 87.

With the surface-emission laser diode 120 of Example 5, negativecarriers and positive carriers are injected respectively from the n-sideelectrode 88 and the p-side electrode 89, and the laser beam of thewavelength of 850 nm is emitted in the direction perpendicular to thesubstrate 81 via the opening formed in the upper electrode 88.

In the surface-emission laser diode such as the one shown in FIG. 5,there is a need of providing the AlAs layer for selective oxidation 84at the side closer to the substrate as compared with the activestructural part 85 in the event the semiconductor layers at thesubstrate side of the active layer 85B in the active structural part 85are formed to have the p-conductivity type. This is because, withcompound semiconductors, mobility is smaller in the p-conductivity typelayer as compared in the n-conductivity type layer and largerconfinement effect is obtained by providing the current confinementstructure in the p-type conductive region.

As noted above, when forming the semiconductor layers at the side of thesubstrate in the p-conductivity type, there is a need of more preciseetching control. With the stacked film construction of Example 5, it ispossible to carry out stable mesa etching by monitoring the progress ofthe etching by using the plasma atomic emission spectrometry, even inthe case the surface-emission laser diode use semiconductor layers ofp-conductivity type at the side closer to the substrate.

Other features of the present invention are similar to those of Example1 explained before, and description thereof will be omitted.

Example 6

Next, a surface-emission laser diode according to Example 6 of thepresent invention will be explained.

FIGS. 6 through 7 show the construction of a surface-emission laserdiode 160 according to Example 6 of the present invention, wherein FIG.7 is an enlarged view showing a region A in the vicinity of the activelayer of the surface-emission laser diode 160 of FIG. 6. It should benoted that the surface-emission laser diode of Example 6 oscillates atthe wavelength of 780 nm.

Referring to FIG. 6, the surface-emission laser diode 160 includes, onan n-(100)GaAs substrate 101 having a surface orientation inclined in adirection of (111)A surface with an inclination angle 15°, a first lowersemiconductor DBR (lower first reflector) 102 formed of a periodicstructure in which an n-AlAs low refractive index layer and ann-Al_(0.3)Ga_(0.7)As high refractive index layer are stacked alternatelyfor 30.5 periods with the thickness corresponding to ¼ times theoscillation wavelength in the medium; and a second lower semiconductorDBR (lower second reflector) 103 formed of a periodic structure in whichan n-Al_(0.9)Ga_(0.1)As low refractive index layer and ann-Al_(0.3)Ga_(0.7)As high refractive index layer are stacked alternatelywith the thickness corresponding to ¼ times the oscillation wavelengthin the medium. It should be noted that FIG. 6 omits detailedillustration.

In the first lower semiconductor DBR 102 and the second lowersemiconductor DBR 103, there is imposed a compositional graded layer ofthe thickness of 20 nm and having the Al content changed gradually fromone value to the other value between every layer that constitutes theDBR, and the thickness including the graded layer is set to be equal to¼ times the oscillation wavelength in the medium. With such aconstruction, the band discontinuity between the high refractive indexlayer and the low refractive index layer is relaxed at the time ofcausing to flow current to the DBR, and the resistance of DBR can bereduced.

Further, on the second layer semiconductor DBR 103, there are stackedconsecutively, an (Al_(0.7)Ga)_(0.5)In_(0.5)P lower first spacer(cladding) layer 104A that achieves lattice matching to the second layersemiconductor DBR 103; a Ga_(0.5)In_(0.5)P lower second spacer layer104B that achieves lattice matching to the (Al_(0.7)Ga)_(0.5)In_(0.5)Plower first spacer (cladding) layer; a quantum well active layer 104C inwhich three GaInPAs quantum well layers 104 a having a compressivestrain composition and a bandgap wavelength of 780 nm and twoGa_(0.5)In_(0.5)P barrier layers 104 b of a lattice-matching compositionto the substrate are stacked alternately; a Ga_(0.5)In_(0.5)P uppersecond spacer layer 104D; and an (Al_(0.7)Ga)_(0.5)In_(0.5)P upper firstspacer (cladding layer) 104E.

The semiconductor layers 104A-104E form a cavity 104 of one wavelength,and there is formed, on the cavity 104, an upper semiconductor DBR(upper reflector) 105 of a periodic structure in which ap-Al_(x)Ga_(1-x)As (x=0.9) low refractive index layer and ap-Al_(x)Ga_(1-x)As (x=0.3) high refractive index layer are stackedalternately for 25 periods, for example is (details are omitted in FIG.6). With the present example, it should be noted that a compositionalgraded layer is interposed between the low refractive index layer andthe high refractive index layer also in the upper reflector 105,similarly to the lower reflector 102 and 103.

Further, a p-GaAs contact layer 106 is formed on the upper reflector105. As explained before, there is formed a cavity of one oscillationwavelength (so-called lambda cavity) between the lower reflector 103 andthe upper reflector 105 with the present embodiment.

Hereinafter, the fabrication process of the surface-emission laser diode160 of Example 6 will be explained.

With the present embodiment, the growth of the semiconductor layers102-106 is conducted by an MOCVD process, wherein TMG (trimethylgallium), TMA (trimethyl aluminum), TMI (trimethyl indium), PH₃(phosphine), and AsH₃ (arsine) are used for the source according to theneeds. Further, H₂Se (hydrogen selenide) is used for the n-type dopantand CBr₄ is used for the p-type dopant. Further, H₂ is used for thecarrier gas.

Because it is possible to form the construction such as a compositionalgraded layer by controlling the supply amount of the source gases, MOCVDprocess is particularly suitable for the crystal growth method of asurface-emission laser diode that includes DBR. Further, it does notrequire high vacuum state as in the case of MBE process and it issufficient to merely control the supply flow rate of the source gasesand the supply time, MOCVD process is suited for mass production. WithExample 6, it should be noted that a part of the low refractive indexlayer of the p-side DBR (upper reflector) 105 close to the active layer104C is formed by an AlAs layer 107.

After formation of such a VCSEL stacked structure, a mesa etching isconducted to a predetermined depth, such that at lease a sidewallsurface of the p-AlAs layer 107 is exposed, and with this, a mesastructure M is formed. Further, the AlAs layer 107 exposed by the mesastructure M is oxidized in water vapor ambient, starting from theexposed sidewall surface, and there is formed an Al_(x)O_(y) currentconfinement part 107A in the AlAs layer 107.

Further, the space surrounding the mesa structure M formed by the mesaetching is filled with the polyimide film 108 for planarization, andthereafter, the polyimide film 108 is removed from the p-contact layer106 and the upper reflector in correspondence to a predetermined opticalexit part 109A, and a p-side electrode 109 is formed on the p-contactlayer 106 so as to avoid the optical exit part 109A. Further, an n-sideelectrode 110 is formed on the rear side of the substrate 101.

With Example 6, current confinement is achieved by selective oxidationof the layer for selective oxidation 107 that contains Al and As as themajor components, and with this, threshold current is reduced. With thecurrent confinement structure that uses an Al oxide film 107A formed byselective oxidation of the layer for selective oxidation 107, it becomespossible to form the current confinement layer 107A near the activelayer 104C, and diffusion of the injected holes is suppressed, and itbecomes possible to efficiently confine the carriers into a minuteregion not exposed to the air. Further, with such a current confinementstructure, there occurs a decrease of refractive index when the AlAsfilm 107 is oxidized to form the Al oxide film 107A, and it becomespossible to confine the light to the minute region where the carriersare confined by way of convex lens effect. Thereby, efficiency ofstimulated emission is improved significantly, and with this, thethreshold current is decreased. Further, such a construction can easilyform a current confinement structure, and it becomes possible to reducethe production cost of the surface-emission laser diode.

Further, with the surface-emission laser diode 160 of Example 6, whichuses AlGaInP for a part of the spacer layers 104A and 104E and GaInPAsfor the barrier layer and the quantum well active layer, and forms thesemiconductor layers on the (100) GaAs substrate of the surfaceorientation inclined in the (111)A direction by 15°, it becomes possibleto suppress the narrowing of bandgap associated with formation ofnatural superlattices, deterioration of surface morphology caused byhillock (hill-like defect) formation, or the effect of non-opticalrecombination centers.

Further, with the surface emission laser diode 160, it should be notedthat (Al_(0.7)Ga)_(0.5)In_(0.5)P, which is a widegap material, is usedfor the spacer layer (cladding layer) 104A and 104E. Because of this,the bandgap difference between the spacer layers 104A and 104E and thequantum well active layer 104 a increases to 743 mV from 466 meV (Alcontent 0.6) for the case the spacer layer is formed of AlGaAs. Further,the surface-emission laser diode 160 is advantageous with regard to thebandgap difference between the barrier layer 104 b and the quantum wellactive layer 104 a, and excellent carrier confinement is realized.

Further, because the quantum well active layer 104 a accumulates thereina compressive strain, it is also attained the increase of gain caused byband splitting of heavy holes and light holes. With these, thesurface-emission laser diode 160 has a very high gain and it is possibleto perform high output power operation at low threshold.

Further, because the quantum well active layer 104 a and the barrierlayer 104 b are formed of the material free from Al, incorporation ofoxygen into these layers is reduced, and it becomes possible to suppressformation of non-optical recombination centers. With this, it becomespossible to realize a surface-emission laser diode of long lifetime.

Further, with the surface-emission laser diode 160 of the presentexample, control of polarization direction is achieved by utilizing theoptical gain anisotropy caused by inclination of the substrate.

When compared with the surface-emission laser diode that uses the (311)Bsubstrate (inclination angle 25°), which is thought most promising atthe present juncture, the surface-emission laser diode of the presentembodiment uses a small inclination angle (15°), and thus, the opticalgain anisotropy becomes inevitably small. Thus, with thesurface-emission laser diode 160 of Example 6, this decrease of theoptical gain anisotropy is compensated for by the increase of theoptical gain anisotropy caused by providing a compressive strain to thequantum well active layer. With such a construction, it becomes possibleto realize a satisfactory polarization control.

Thus, according to Example 6 of the present invention, the gain of theactive layer 104 a is increased and heat dissipation is improved, andwith this, it becomes possible to realize a high output powersurface-emission laser diode of 780 nm wavelength band having lowthreshold value for laser oscillation, good reliability and controlledpolarization direction.

It should be noted that the foregoing effect of the present inventiondecreases at short wavelength band, but it appears conspicuously in thewavelength band longer than 680 nm. For example, when compared with thesurface-emission laser diode of the 780 nm band that uses the activelayer of AlGaAs/AlGaAs system, it should be noted that the bandgapdifference between the Al_(x)Ga_(1-x)As (x=0.6, Eg=2.0226 eV) having thelargest bandgap in the typical compositional range used forAl_(x)Ga_(1-x)As (0<x≦1) system spacer layer and the active layer of thecompositional wavelength 780 nm (Eg=1.5567 eV), is generally equal tothe bandgap difference (460 meV) between the(Al_(a)Ga_(1-a))_(b)In_(1-b)P (a=0.7, b=0.5, Eg=2.289 eV) having thelargest bandgap in the typical compositional range used for the(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) spacer layer and the activelayer of the compositional wavelength of 680 nm (Eg=1.8233 eV).

Further, with regard to the bandgap difference between the barrier layerand the quantum well active layer, the bandgap different to the activelayer of the compositional wavelength of 680 nm becomes about 200 meVwhen the barrier layer is formed of Ga_(e)In_(1-e)P_(f)As_(1-f) (e=0.6,f=1, Eg=2.02 eV), while this is generally equivalent to the case of thesurface-emission laser diode of 780 nm that uses the AlGaAs/AlGaAssystem active layer.

This means that it becomes possible to achieve the carrier confinementequivalent to or superior to the case of the surface-emission laserdiode of the 780 nm band that uses the active layer of the AlGaAs/AlGaAssystem while using an Al-free active layer (quantum well active layerand barrier layer) by using the spacer layer of the AlGaInP system,provided that the compositional wavelength is longer than 680 nm.Further, by taking into consideration the effect of the strained quantumwell active layer, it becomes possible with the surface-emission laserdiode of the present example to have the characteristics equal to orsuperior to the surface-emission laser diode of the 780 nm band thatuses the conventional active layer of the AlGaAs/AlGaAs system.

Example 7

Next, a surface-emission laser diode according to Example 7 of thepresent invention will be explained.

FIG. 8 is a top view diagram of the surface-emission laser diode 180 ofExample 7. The surface-emission laser diode 180 of Example 7 has across-sectional structure identical to that of the surface-emissionlaser diode 160 of Example 6, except that the mesa structure M thereofis formed with anisotropy such that the mesa structure M forms anelongated circular shape in the (111)A direction when viewed from thelight emission direction of the surface-emission laser diode. Thus,explanation about the cross-sectional structure of the surface-emissionlaser diode 180 is omitted. Further, because the mesa structure has theelongated elliptic shape, the current injection region 107 defined bythe Al oxide film 107A also has an elongated shape in the direction ofthe (111)A surface. It should be noted that the foregoing shapeanisotropy is not limited to elongated elliptic shape but it is possibleto use other shape such as rectangular shape.

The present embodiment uses optical gain anisotropy caused by theinclination of the substrate 101 for the polarization control. However,the inclination angle is small (15°) as compared with the case of usingthe (311)B substrate (inclination angle 25°, which is thought mostpromising at the current juncture, for the polarization control, andthus, the obtained optical gain anisotropy becomes inevitably small.

With Example 7, such decrease of the optical gain anisotropy iscompensated for by increasing the optical gain in the inclinationdirection of the substrate (direction of (111)A surface), by increasingthe optical gain anisotropy by providing a compressive strain to thequantum well active layer 104 a and further by providing anisotropy tothe outer shape of the active layer as viewed from the optical emissiondirection of the surface-emission laser diode 180, more specifically, byusing an elongated shape in the direction of the (111)A surface.Thereby, polarization control comparable to the case of using the (311)Bsubstrate is realized.

Example 8

Next, a surface-emission laser diode of Example 7 of the presentinvention will be explained.

FIG. 9 shows the construction of surface-emission laser diode 200 ofExample 7.

The surface-emission laser diode 200 of FIG. 9 has a constructionsimilar to that of the surface-emission laser diode of Example 6,wherein it should be noted that FIG. 9 shows the region around theactive layer of the surface-emission laser diode of Example 8 inenlarged scale.

Referring to FIG. 9, the surface-emission laser diode 200 of Example 8is different from the surface-emission laser diode 160 of Example 6 inthe point that Ga_(0.6)In_(0.4)P having a tensile strain is used for thebarrier layer 104 b. Further, with Example 8, the barrier layer ofGa_(0.6)In_(0.4)P having the tensile strain is provided also under thefirst quantum well active layer 104 a and over the third quantum wellactive layer 104 a. Other structure is identical to the surface-emissionlaser diode 160 of FIG. 6.

When comparison is made for the GaInPAs system having the same latticeconstant, GaInP has the largest bandgap. Further, in the case of usingGaInPAs for the barrier layer 14 b, it becomes possible to secure alarge bandgap when the composition of small lattice constant is used.Thereby, the band discontinuity to the quantum well active layer 104 acan be increased further, and it becomes possible to obtain furtherlow-threshold operation and high output power operation. For example,the tensile-strained Ga_(0.4)In_(0.4)P layer used with Example 8 has abandgap of 2.02 eV, while this bandgap is larger than that of aGa_(0.5)In_(0.5)P lattice matched layer of Example 6, which has thebandgap of 1.87 eV, by 150 meV.

Example 9

FIG. 10 is a top view diagram of a surface-emission laser diode array220 according to Example 9 of the present invention.

Referring to FIG. 10, there are provided ten surface-emission laserdiodes 200 of Example 8 on the substrate 101 with the surface-emissionlaser diode array 220 in one-dimensional array, except that, in Example9, p-side and n-side of the surface-emission laser diode 220 areexchanged. Thus, with Example 9, the GaAs substrate 101, on which thesurface-emission laser diode 220 is formed, is a substrate, and thereare provided n-side electrode pads 109 on the top surface and a p-sidecommon electrode 110 is provided on the rear side.

While plural surface-emission laser diodes 200 are arranged with theexample of FIG. 10 in one-dimensional array, it is possible to arrangesuch surface-emission laser diodes in two dimensional array.

Example 100

FIG. 11 shows an optical transmission module 240 of Example 10.

Referring to FIG. 11, the optical transmission module 240 has aconstruction of combining low-cost acrylic POFs (plastic optical fibers)240 with the surface-emission laser diode array chip of FIG. 10. Thusthe laser beam 241A from each surface emission laser diode 200 isinjected into a corresponding POF 241 and transmitted therethrough.

An acrylic POF shows the minimum absorption loss at the wavelength of650 nm, and thus, the use of surface-emission laser diode of 650 nm hasbeen studied. However, conventional surface-emission laser diode of 650nm band has poor high temperature characteristics, and practical usethereof has not been successful. In view of these circumstances, LEDshave been used with the optical transmission modules that use such POF.However, it is difficult to conduct high-speed modulation with LED, andit is thought indispensable to use a laser diode for achievinghigh-speed transmission exceeding 1 Gbps.

With the surface-emission laser diode 200 used with the opticaltransmission module of Example 10, the laser oscillation wavelength is780 nm. However, the surface-emission laser diode has excellent heatdissipation characteristics and large active layer gain, and it ispossible to perform high output power operation. Further, because it hasexcellent high temperature characteristics, it is possible to conductoptical transmission for short distance, in spite of the fact that thereis a problem of absorption loss by POF.

In the field of optical communication, parallel transmission isattempted for transmitting larger amount of data at the same time byusing a laser array in which plural laser diodes are integrated. Withthis, high-speed parallel transmission becomes possible and it becomespossible to transmit larger amount of data at the same time.

With Example 10 shown in FIG. 11, the surface-laser diode elements 100in the surface-laser diode array and the optical fibers 241 are providedin one to one correspondence, while it is also possible to increase thetransmission speed further by conducting wavelength multiplexedtransmission via a single optical fiber by arranging pluralsurface-emission laser diode elements of different wavelengths in theform of one-dimensional or two-dimensional array.

Further, the present example combines the low-cost surface-emissionlaser diode element 200 with low-cost POF 241, and thus, it becomespossible to realize a low-cost optical transmission module, and withthis, it becomes possible to realize a low-cost optical communicationsystem. Because such optical communication system is extremely low cost,it is useful for short-distance data communication in homes, in officerooms and inside of apparatuses.

Example 11

FIG. 12 shows the construction of an optical transceiver module 260according to Example 11 of the present invention.

Referring to FIG. 12, the optical transceiver module 260 has aconstruction that combines the surface-emission laser diode element 200of Example 8, a photodiode 261 for reception and an acrylic POF 262.

Both of the surface-emission laser diode 200 of the present inventionand POF 262 are low cost, and it becomes possible, with the opticaltransceiver system 260 that uses the surface-emission laser diodeelement 200 of the present invention for the optical communicationsystem, to realize a low-cost optical communication system byconstructing the optical transceiver module by combining thesurface-emission laser diode element 200 for transmission and thephotodiode 261 for reception, with a single POF 262.

Further, in view of the fact that POF has a large fiber diameter and itis possible to reduce the mounting cost because of the easiness ofcoupling to a fiber, it becomes possible to reduce the cost of theoptical transceiver module 260 further. Further, in view of the factthat the surface-emission laser diode 200 of the present invention hasexcellent temperature characteristics and low laser oscillationthreshold, the amount of heat generation is small and it becomespossible to realize a low-cost optical communication system that can beused up to high temperatures without cooling.

The optical communicating system 260 according to the present embodimentis particularly useful for short-distance communication such as theoptical transmission between devices such as computers by way of LAN(Local Area Network) that uses POF, the optical communications betweenLSIs on a board, the optical interconnection between elements inside anLSI, and the like.

Meanwhile, the processing performance of LSIs is increasing recently,and it is predicted that the transmission speed of the part connectingsuch LSIs becomes the bottleneck problem.

On the other hand, by changing the signal connection inside the systemfrom electrical connection to optical interconnect, and by using theoptical transmission module 240 or the optical transceiver module 260for connecting the boards of a computer system, the LSIs in a board orelements inside an LSI, it becomes possible to realize a super-fastcomputer system.

Further, by using the optical transmission module 220 or the opticaltransceiver module 260 of the present invention for connection pluralcomputer systems, it becomes possible to construct a super-fast networksystem. Particularly, the power consumption can be reduced significantlywith the surface-emission laser diode as compared with the edge-emissionlaser diode, and because of easiness of forming a two-dimensional arraywith the use of surface-emission laser diode elements, the presentinvention is particularly suited for constructing an opticalcommunication system of parallel transmission type.

Example 12

FIG. 13 shows the construction of a laser printer 280 according toExample 12, wherein it should be noted that the laser printer 280 ofFIG. 13 combines a surface-emission laser diode array chip (16 beamVSCEL array) 281, in which the surface-emission laser diodes 200 ofExample 8 having the oscillation wavelength of 780 nm are arranged onthe GaAs substrate 101 in the form of a 4×4 two-dimensional array, witha photosensitive drum 282. FIG. 13 particularly shows the outline ofoptical scanning part of the laser printer 280.

FIG. 14 is a top view showing the outline construction of thesurface-emission laser diode array chip (16 beam VCSEL array) 281 usedwith the laser printer 280 of FIG. 13. With such a surface-emissionlaser diode array chip (16 beam VCSEL array) 281, it is possible torealize the situation equivalent to the case the optical sources arealigned on the photosensitive body 282 in the sub-scanning directionV-SCAN with an interval of 10 μm as shown in FIG. 14, by adjusting thetiming of turning-on.

With Example 12, the plural optical beams from the pluralsurface-emission laser diode array 200 are focused upon a scanningpolygonal mirror 284 via an optical system including a lens 283 and formplural optical spots aligned in the sub-scanning direction V-SCAN byadjusting the timing of turning-on while rotating the polygonal mirror284 at high speed. The plural optical beams are then focused upon thephotosensitive body 282 serving for the scanning surface via a so-calledf-θ lens for image formation. Thus, according to the present embodiment,the image formation is made by scanning plural beams at the same time.

According to Example 12, it becomes possible to carry out opticalwriting on the photosensitive body 282 with an interval of about 10 μmin the sub-scanning direction V-SCAN, while this corresponds to theresolution of 2400 DPI (dot/inch). On the other hand, the writinginterval in the main scanning direction HSCAN can be controlled easilyby the turn-on timing of the optical source.

With the laser printer 280 of the present embodiment, it becomespossible to write 16 dots at the same time, and it becomes possible toperform high-speed printing. Further, by increasing the number of thesurface-emission laser diodes 200 in the array, a further high speedprinting becomes possible. Further, by adjusting the interval betweenthe surface-emission laser diode elements 200, the dot interval in thesub-scanning direction H-SCAN can be adjusted, and it becomes possibleto carry out high-quality printing with the density higher than 2400DPI. With the surface-emission laser diode of Example 12, the efficiencyis improved over the conventional surface-emission laser diode. Further,it has superior heat dissipation characteristics and it becomes possibleto maintain high output power even when plural elements are operated atthe same time. Thereby, it becomes possible to increase the printingspeed.

While Example 12 has shown the application of the laser to laserprinter, the surface-emission laser diode of the present invention isapplicable to other image forming apparatuses. Further, thesurface-emission laser diode of the present invention is applicable alsoto the optical source for recording and playback in CDs, or the like.Thus, the present invention is applicable to an optical pickup system.Further, the present invention is applicable to optoelectronic hybridintegrated circuit, or the like.

In Examples 9-12, it is also possible to use a surface-emission laserdiode of any modes explained before or to be explained below in place ofthe surface-emission laser diode 200.

(Twelfth Mode)

Hereinafter, twelfth mode of the present invention will be explainedwith reference to FIGS. 15 and 16, wherein it should be noted thetwelfth mode of the present invention relates to the principle of theconstruction and operation of the surface-emission laser diode.

(1) First Construction

The surface-emission laser diode according to the first construction ofthe twelfth mode of the present invention comprises a GaAs substrate; acavity region formed over said GaAs substrate, said cavity regionincluding at least one quantum well active layer producing a laser lightand barrier layers; and an upper reflector and a lower reflectorprovided at a top part and a bottom part of said cavity region over saidGaAs substrate, said upper reflector and/or said lower reflectorincluding a semiconductor Bragg reflector, at least a part of saidsemiconductor distributed Bragg reflector comprising a semiconductorlayer containing Al, Ga and As as major components, wherein there isprovided, between said active layer and said semiconductor layer thatcontains Al, Ga and As as major components, a semiconductor layercontaining Al, In and P as major components such that said semiconductorlayer containing Al, In and P as major components is provided adjacentto said semiconductor layer that contains Al, Ga and As as majorcomponents, an interface between said semiconductor layer containing Al,Ga and As as major components and said semiconductor layer containingAl, In and P as major components being formed coincident to a locationof a node of electric field strength distribution.

With the surface-emission laser diode of the first construction, thesemiconductor layer containing Al, In and P as the major components isformed, for example by (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1).

In conventional surface-emission laser diodes, the interface between thecavity region and the reflector is located coincident to the anti-nodeof the electric field strength distribution and the semiconductor layercontaining Al, In and P as the major components is provided at theuppermost part of the cavity region. Thus, the interface between to theupper reflector formed of the semiconductor layer containing Al, Ga andAs as the major components is formed at the location where the effect ofoptical absorption appears most conspicuously. Meanwhile, whenconducting crystal growth of a semiconductor layer containing Al, Ga andAs as the major components on the semiconductor layer containing Al, Inand P as the major components, there is a tendency that segregation ofIn such as In carry over takes is place, and thus, there has been a needof suppressing this problem. Conventionally, it has been difficult toavoid increase of the threshold value. It should be noted that thisproblem appears conspicuous when conducting crystal growth of asemiconductor layer containing Al, Ga and As as the major components, onthe semiconductor layer containing Al, In and P as the major components.

Contrary to the foregoing, with the surface-emission laser diode of thefirst construction, in which the interface 3 between the semiconductorlayer 1 containing Al, In and P as the major components and thesemiconductor layer 2 (part of the upper reflector) containing Al, Gaand As as the major components, is located at the node N of the electricfield strength distribution, the surface-emission laser diode isdesigned so as to drastically decrease the effect of the opticalabsorption at the interface 3, and it becomes possible to suppress theincrease of laser oscillation threshold value effectively even whenthere has been caused some segregation of In. In FIG. 1, it should benoted that the anti-node of the electric field strength distribution isrepresented by N.

Here, it is further advantageous to provide a thin layer for Insegregation suppression between the semiconductor layer 1 containing Al,In and P as the major component and the semiconductor layer 2 (part ofthe upper reflector) containing Al, Ga and As as the major components,for the purpose of suppressing In segregation.

For the semiconductor layer 1 that contains Al, In and P as the majorcomponents, it is possible to use a (Al_(a)Ga_(1-a))_(b)In_(1-b)P(0<a≦1, 0≦b≦a) layer. With such a construction, the effect of decreaseof the threshold value is attained for the surface-emission laser diodethat uses AlGaInP layer, irrespective of the wavelength, and thus, notonly in the red color surface-emission laser diode of the 650 nm band,in which the use of AlGaInP layer is essential, but also in thesurface-emission laser diodes of the 780 nm band, 850 nm band, or thelike.

(2) Second Construction

The surface-emission laser diode according to the second construction ofthe second mode of the present invention comprises: a GaAs substrate; acavity region formed over said GaAs substrate and having at least onequantum well active layer producing a laser light and barrier layers;and an upper reflector and a lower reflector provided at a top part anda bottom part of said cavity region over said GaAs substrate, said upperreflector and/or lower reflector including a semiconductor distributedBragg reflector, at least a part of said semiconductor distributed Braggreflector comprising a semiconductor layer containing Al, Ga and As asmajor components, there being provided, between said active layer andsaid semiconductor layer containing Al, Ga and As as major components, a(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer adjacent to saidsemiconductor layer containing Al, Ga and As as major components, said(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer being added with Mg(magnesium) as a p-type dopant, said semiconductor layer containing Al,Ga and As as major components being added with C (carbon) as a p-typedopant.

For the p-type dopant of the semiconductor layer containing Al, In and Pas the major components, Zn (zinc) is used commonly, while Zn has alarge diffusion coefficient and tends to cause diffusion to the activelayer or near the active layer, and there arises the problem ofdegradation of the device characteristics, such as decrease ofefficiency of photoemission caused by degradation of the crystalquality, or increase of the absorption loss.

Contrary to this, Mg, which can be used for the p-type dopant, ischaracterized by smaller diffusion coefficient as compared with Zn andthe foregoing problem can be improved. On the other hand, in thesemiconductor layer that contains Al, Ga and As as the major components,C has an even smaller diffusion coefficient. Further, addition of Mg tothe material containing As causes the problem of poor controllability ofdoping caused by memory effect.

Thus, with the present construction, Mg is added primarily to thesemiconductor layer containing Al, In and P as the major components andC is added to the semiconductor layer containing Al, Ga and As for themajor components. With this, diffusion of dopant or memory effect issuppressed, and the controllability of doping is improved, and thedoping profile close to the designed profile is obtained.

Further, degradation of crystal quality of the active layer issuppressed and high output power operation and low threshold value arerealized.

(3) Third Construction

The surface-emission laser diode according to the third construction ofthe twelfth mode of the present invention comprises: a GaAs substrate; acavity region formed over said GaAs substrate, said cavity regionincluding at least one quantum well active layer producing a laser lightand barrier layers; and an upper reflector and a lower reflectorprovided at a top part and a bottom part of said cavity region over saidGaAs substrate, said upper reflector and/or lower reflector including asemiconductor distributed Bragg reflector, at least a part of saidsemiconductor distributed Bragg reflector comprising a semiconductorlayer containing Al, Ga and As as major components, there beingprovided, between said active layer and said semiconductor layercontaining Al, Ga and As as major components, a(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer adjacent to saidsemiconductor layer containing Al, Ga and As as major components, said(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) layer being a semiconductorlayer formed of a short period superlattice structure of AlInP andGaInP.

Although it changes depending on the to material, thermal resistance ofa semiconductor layer increases with increasing number of the elementsthat constitutes the semiconductor material. Thus, AlGaInP, a quaternarymaterial, has a large thermal resistance. Thus, the heat generated insuch an active layer is not dissipated easily but is accumulated in theactive layer and invites the temperature rise of the active layer. Thus,there has been a problem that the optical output saturates with smallinjection current.

In the case of surface-emission laser diode, it can be regarded, whenthere is formed a superlattice structure by stacking the layers having athickness sufficiently smaller than the oscillating wavelengthalternately, that it is optically equivalent to the case in which thereis formed a mixed crystal having an average composition by mixing theselayers uniformly, although there is a report that there occurs slightincrease of refractive index by forming the superlattice structure.Thus, it is possible to construct a reflector by using the semiconductorlayers thus constructed by the superlattice structure.

Here, in view of the fact that the thermal resistance of the ternarysystem material such as AlInP or GaInP is smaller than that of AlGaInP,a quaternary material, and because of the fact that AlInP or GaInP canbe lattice-matched to the GaAs substrate similarly to the case ofAlGaInP, the present construction decreases the thermal resistance byforming a superlattice structure in place of the conventional AlGaInPsemiconductor layer of uniform composition, by choosing at least twomaterials of small thermal resistance as compared with the averagecomposition. With this, the heat generated in the active layer isefficiently dissipated, and temperature rise of the active layer bycurrent injection is decreased. Further, it becomes possible to conductthe current injection with a higher level than conventional case, whilethis leads to higher output, and it becomes possible to obtain asurface-emission laser diode capable of operating at high output power.

(4) Fourth Construction

The surface-emission laser diode according to the fourth construction ofthe twelfth mode comprises: a GaAs substrate; a cavity region formedover said GaAs substrate, said cavity region including at least onequantum well active layer producing a laser light and barrier layers;and an upper reflector and a lower reflector provided at a top part anda bottom part of said cavity region over said GaAs substrate, said upperreflector and/or lower reflector including a semiconductor distributedBragg reflector, at least a part of said semiconductor distributed Braggreflector comprising a low refractive index layer of Al_(x)Ga_(1-x)As(0<x≦1) and a high refractive index layer of Al_(y)Ga_(1-y)As (0≦y<x≦1),one of said low refractive index layers constituting said upperreflector and/or said lower reflector and located closest to said activelayer comprising (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1), aninterface between said cavity region and said low refractive index layerof said upper reflector and/or said lower reflector located closest tosaid active layer being coincident to an anti-node of an electricstrength distribution.

FIG. 16 shows the construction of the surface-emission laser diodeaccording to the fourth construction.

Referring to FIG. 16, the surface-emission laser diode of the fourthconstruction is constructed such that the interface 306 between thecavity region 304 including the active layer 307 and the upper reflector305 is located coincident to the anti-node AN of the electric fieldstrength distribution in accordance with the conventionally usedconstruction, while it should be noted that, with the construction ofFIG. 2, the interface 309 for the case of growing the semiconductorlayer (upper reflector 305) that contains Al, Ga and As as the majorcomponents on the foregoing (Al_(a)Ga_(1-a))_(b)In_(1-b)P layer 308 ismade coincident to the node N of the electric field strengthdistribution by forming the low refractive index layer (λ/4 thickness)308 of the upper reflector 305 closest to the active layer 307 with(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1). With this, the effect ofoptical absorption at the interface 309 is decreased significantly, andit becomes possible to suppress the increase of the threshold valueeffectively even in the case there is caused some segregation of In. InFIG. 16, the reference numeral 310 forms (a part of) the lowerreflector. Further, in FIG. 16, only a part of the upper reflector 305is illustrated.

In the construction of FIG. 16, it is possible to construct the lowerreflector 310 by a first lower reflector that uses AlAs for the lowrefractive index layer, a second lower reflector that usesAl_(x1)Ga_(1-x1)As (0<x₁<1), and at least one(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive index layer,consecutively from the side of the substrate.

The thermal resistance of AlAs increases sharply when only a smallamount of Ga is added. Contrary to this, the lower reflector 310includes AlAs having a small thermal resistance for the low refractiveindex layer, and thus, dissipation of the heat generated in the activelayer 307 is improved and temperature rise at the time of driving issuppressed. Thereby, a high output power surface-emission laser diode ofexcellent temperature characteristics is obtained. In the case of thestructure that includes the current confinement structure that uses anAl oxide film, it becomes possible to stop the etching for mesaformation between the layer for selective oxidation, which is to beconverted to the Al oxide film, and AlAs of the first lower reflector,by providing a second lower reflector of an AlGaAs low refractive indexlayer of small Al content between the first lower reflector that usesthe AlAs low refractive index layer and the layer containing In.

Further, in the case a high refractive index layer of Al_(y)Ga_(1-y)As(0≦y<x≦1) and a low refractive index layer of(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) are to be stacked in theupper reflector 305 or the lower reflector 310, it becomes possible toprovide an (Al_(a1)Ga_(1-a1))_(b1)In_(1-b1)P (0≦a₁≦1, 0≦b₁≦1)intermediate layer (In segregation suppressor layer) at such aninterface with Al content smaller than the (Al_(a)Ga_(1-a))_(b)In_(1-b)P(0<a≦1, 0≦b≦1) low refractive index layer.

By inserting such intermediate layer of small Al content when stackingthe AlyGa1−yAs (0≦y<x≦1) high refractive index layer on the(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive index layer,the Al content at the interface is decreased, it becomes possible toform the AlyGa1−yAs high refractive index layer on the(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive index layereasily over a wide process condition.

Further, at the heterojunction between the material of AlGaAs system andthe material of the AlGaInP system, there occurs an increase of banddiscontinuity in the valence band when the Al content of the material ofthe AlGaInP system is large. Because the intermediate layer of small Alcontent is inserted, the band discontinuity of the valence band isdecreased, and it becomes possible to decrease the electric resistancein the stacked direction.

Further, with the semiconductor distributed Bragg reflector thatconstitutes the p-type reflector in any of the upper reflector 305 orthe lower reflector 310, it is possible to add C (carbon) to theAl_(x)Ga_(1-x)As (0<x≦1) low refractive index layer and theAl_(y)Ga_(1-y)As (0≦y<x≦1) high refractive index layer as a p-typedopant, and add Mg (magnesium) to the (Al_(a)Ga_(1-a))_(b)In_(1-b)P(0<a≦1, 0≦b≦1) low refractive index layer 308 as a p-type dopant.

Further, it is possible that the (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1,0≦b≦1) low refractive index layer 303 may be the semiconductor layerformed of the short period super-lattice structure of AlInP and GaInP.

(5) Fifth Construction

The surface-emission laser diode according to the fifth construction ofthe twelfth mode of the present invention is a surface-emission laserdiode according to the fourth construction, wherein there is provided aspacer layer between the active layer and the upper reflector and/orlower reflector, a part of the spacer layer comprises a(Al_(a′)Ga_(1-a′))_(b′)In_(1-b′)P (0≦a′≦1, 0≦b′≦1) layer, the quantumwell active layer comprises a Ga_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1)having a compressive strain, and the barrier layer is formed ofGa_(a)In_(1-a)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1).

Thus, with the example of FIG. 16, at least the low refractive indexlayer among the low refractive index layers constituting the upperreflector 305 and located closest to the active layer is 307 is formedof an AlGaInP layer, and a material of the GaInPAs system is used forthe barrier layer or the quantum well active layer.

Thus, it becomes possible to use the widegap material such as the(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive index layerand the (Al_(a′)Ga_(1-a)′)_(b′)In_(1-b′)P (0≦a′≦1, 0≦b′≦1) spacer layerfor the cladding layer for carrier confinement, and the bandgapdifference between the cladding layer and the quantum well active layeris increased further as compared with the case in which the claddinglayer for carrier confinement is formed of the material of AlGaAssystem. Here, it should be noted that the spacer layer is the layerprovided between the active layer and the reflector in ordinaryconstruction and performs the function of carrier confinement.

With regard to the carrier confinement, there are cases in which otherlow refractive index layers of the DBR close to the active layer performthe function in addition to the spacer layer. In the fifth construction,the (Al_(a′)Ga_(1-a′))_(b′)In_(1-b′)P (0≦a′1, 0≦b′≦1) spacer layer andthe (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive indexlayer can perform the function of the cladding layer.

Referring to Table 2 explained with the previous mode, for example, itbecomes possible, with the 780 nm surface-emission laser diode of thesystem of AlGaInP (spacer layer)/GaInPAs (quantum well active layer), tosecure a bandgap difference between the spacer layer and the quantumwell layer, and between the barrier layer and the quantum well layer,larger than the 850 nm surface emission laser that uses theAlGaAs/AlGaAs system, by using an (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)Player of the bandgap Eg of 2.289 eV for the spacer layer, a GaInPAslayer having the bandgap of 1.5567 eV and accumulating a compressivestrain for the quantum well layer, and further by using Ga_(x)In_(1-x)Phaving a bandgap Eg of accumulating a tensile strain for the barrierlayer.

Further, referring to Table 2, there occurs an increase of bandsplitting between the heavy holes and the light holes by forming theactive layer 307 by the quantum well layer of compressive straincomposition as explained with reference to the previous modes, and thus,there occurs an increase of gain and decrease of the threshold value,and the laser oscillation efficiency is increased together with thelaser output. It should be noted that this effect is not attained withthe 850 nm surface-emission laser diode of the AlGaAs/AlGaAs system.

Thus, with the fifth construction of the twelfth mode of the presentinvention, a high-efficiency and high output power surface-emissionlaser diode is obtained with the threshold value lower than that of the850 nm surface-emission laser diode of the AlGaAs/AlGaAs system.Further, with the present construction, there occurs an increase ofcarrier confinement efficiency, and the threshold value is decreasedalso by the increase of gain attained by the use of the strained quantumwell active layer. With this, it becomes possible to reduce thereflectivity of the DBR 305 at the optical exit side. Thereby, theoutput of the surface emission laser can be increased further.

Further, by constructing the quantum well layer in the quantum wellactive layer 307 by the Ga_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1)layer, and by constructing the barrier layer in the quantum well activelayer 307 by Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1), it becomespossible to form the active layer 307 by the material free from Al. Withthis, incorporation of oxygen into the active layer 307 is reducedsimilarly to the second mode explained before, and formation of thenon-optical recombination centers is suppressed, and a long lifetimesurface-emission laser diode is realized.

Thus, according to Fifth Construction of the present mode, it becomespossible to realize a reliable high output surface-emission laser diodeoperating at the wavelength shorter than 850 nm, having large gain forthe active layer and reduced threshold value, by using the AlGaInPmaterial for a part of the spacer layer and by using GaInPAs for thebarrier layer and the quantum well active layer.

While the fifth construction limits the wavelength to be shorter than850 nm, this is merely because advantage over the conventionalconstruction appears extremely conspicuously in this wavelength range,and the foregoing advantageous effect is attained more or less in thewavelength longer than 850 nm.

Further, with the fifth construction, too, the bandgap of GaInP is thelargest among the materials of the GaInPAs system used for the barrierlayer in the quantum well active layer of the surface-emission laserdiode when comparison is made on the basis of the same lattice constant.Further, larger bandgap is secured with the material of the smallerlattice constant. Thus, by using the material of the GaInP system ofsmall lattice constant for the barrier layer, it becomes possible forthe surface-emission laser diode to perform high output power operationwith low threshold value. For example, it should be noted that thebandgap of a Ga_(0.6)In_(0.4)P tensile-strained layer is 2.02 eV whilethe bandgap of a Ga_(0.5)InP_(0.5)P lattice-matched layer is 1.87 eV.Thus, the bandgap of the Ga_(0.6)In_(0.4)P tensile-strained layer islarger by 150 meV.

While the fifth construction has been explained based on the fourthconstruction that uses AlGaInP layer as the low refractive index layerclosest to the active layer, similar effect as in the case of usingAlGaInP for the cladding layer and the material of GaInPAs system forthe barrier layer and the quantum well active layer, is attained also inthe case of the first through third constructions, in which a part ofthe spacer layer provided between the active layer and the reflector isformed of the AlGaInP layer, the quantum well active layer is formed ofGa_(c)In_(1-e)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1) and the barrier layer isformed of Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1).

Thus, in the third construction, it is possible to construct thesurface-emission laser diode such that there is provided a spacer layerbetween the active layer and the upper reflector and/or the lowerreflector, a part of the spacer layer is formed of the AlGaInP layer,the quantum well active is formed of Ga_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1,0≦d≦1), and the barrier layer is formed of Ga_(e)In_(1-e)P_(f)As_(1-f)(0≦e≦1, 0≦f≦1).

Thus, by using the spacer layer of the composition(Al_(a′)Ga_(1-a′))_(b′)P (0≦a′≦1, 0≦b′≦1), it becomes possible to securea very large bandgap difference between the spacer layer and the quantumwell active layer as compared with the case of forming the spacer layerby the material of the AlGaAs system. Further, by using a compressivestrain composition for the quantum well active layer, the thresholdvalue is decreased and the efficiency of laser oscillation is improved.Thereby, it becomes possible to operate the surface-emission laser diodewith high output power.

Further, as a result of improvement of the carrier confinementefficiency and as a result of decrease of the threshold value attainedby the gain increase caused by the use of the strained quantum wellactive layer, it becomes possible to reduce the reflectivity of the DBR305 at the optical exit side, and it becomes possible to increase theoptical output further. Further, as a result of the use ofGa_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1) for the quantum well activelayer and as a result of the use of Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1,0≦f≦1) for the barrier layer, It becomes possible to realize a longlifetime surface-emission laser diode in which the active layer isformed of the material free from Al and incorporation of oxygen into theactive layer is reduced and formation of the non-optical recombinationcenter is suppressed.

Thus, according to the present construction, a highly reliable highoutput power surface-emission laser diode operating at the wavelength ofshorter than 850 nm is obtained with high active layer gain and lowthreshold value.

(6) Sixth Construction

The surface-emission laser diode according to the sixth construction ofthe twelfth mode of the present invention is a surface-emission laserdiode of the fifth construction, wherein the oscillation wavelength islonger than about 680 nm.

Comparing the surface-emission laser diode with the 780 nmsurface-emission laser diode having the active layer of theAlGaAs/AlGaAs system, it can be seen that the bandgap difference betweenthe Al_(x)Ga_(1-x)As (x=0.6, Eg=2.0226 eV), which provides the largestbandgap in the typical compositional range of the Al_(x)Ga_(1-x)As(0<x≦1) system spacer layer used with the surface-emission laser diodeof the AlGaAs/AlGaAs system, and the active layer of the compositionalwavelength of 780 nm (Eg=1.5567 eV) is generally equal to the bandgapdifference (460 meV) between (Al_(a)Ga_(1-a))_(b)In_(1-b)P (a=0.7,b=0.5, Eg=2.289 eV), which provides the largest bandgap in the typicalcompositional range of the (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1)spacer layer used with the surface emission laser diode of the presentmode and the active layer of the compositional wavelength of 680 nm(Eg=1.8233 eV).

Further, with regard to the bandgap difference between the barrier layerand the quantum well active layer, it should be noted that the bandgapdifference to the active layer of the compositional wavelength of 680 nmbecomes about 200 meV, assuming that the barrier layer has thecomposition Ga_(e)In_(1-e)P_(f)As_(1-f) (e=0.6, f=1, Eg=2.02 eV), whilethis is generally equal to the case of the 780 nm surface-emission laserdiode of the active layer of the AlGaAs/AlGaAs system.

This means, that by using the spacer layer of the AlGaInP system, itbecomes possible to achieve the carrier confinement equivalent or largerthan the surface-emission laser diode of 780 nm that uses theAlGaAs/AlGaAs system active layer, even in the case the surface-emissionlaser diode is the one that uses the Al-free active layer (quantum wellactive layer and the barrier layer), as long as the compositionalwavelength is longer than 680 nm. In practice, the characteristicsequivalent or more are attained in view of the effect of the strainedquantum well active layer.

(7) Seventh Construction

The surface-emission laser diode according to the seventh example of thetwelfth mode of the present invention is a surface-emission laser diodeof any of the first through sixth constructions, wherein the surfaceorientation of the substrate, on which growth is to be made, is formed a(100) surface inclined in the direction of (111)A surface within therange of 5° to 20°.

In the crystal growth of the material that contains Al, In and P orGaInP, it is preferable to use a (100) GaAs substrate having a surfaceorientation inclined in the direction of a (111)A surface by an angle(inclination angle) of the range of 5° to 20° for the substrate 41. Thereason of this is that, when the surface orientation of the substrate isclose to (100), there is caused problems such as decrease of bandgap byformation of natural superlattices, deterioration of surface morphologycaused by hillock (hill-like defect), formation of non-opticalrecombination centers, and the like, and there is a possibility that thedevice characteristics of the laser diode formed on the substrate isadversary affected.

When the surface orientation of the substrate is inclined from (100)surface to the direction of (111)A surface, on the other hand, formationof the natural superlattice is suppressed in correspondence to theinclination angle. Thus, the bandgap changes sharply in the range of theinclination angle of 10° to 15°, and thereafter, the band gap approachesthe nominal bandgap (bandgap value of the mixed crystal). Further,formation of hillocks is gradually suppressed.

On the other hand, when the inclination angle in the direction of (111)Asurface exceeds 20°, crystal growth on the substrate becomes difficult.Thus, with the red color laser (630 nm to 680 nm) that uses the AlGaInPmaterial, the substrate inclined to the angle in the range of 5° to 20°(in many cases in the range of 7° to 15°) is used commonly. This appliesnot only to the case AlGaInP is used for the spacer layer (claddinglayer), but also to the case in which GaInP is used for the barrierlayer as in the example of Table 2. Further, in anticipation ofadversary effect caused also in the case the barrier layer and thequantum well active layer are formed of GaInPAs, it is preferable to usea (100) GaAs substrate having a surface orientation inclined in thedirection of (111)A surface with the angle in the range of 5° to 20°(more preferably with the angle in the range of 7° to 15°), for thegrowth of these materials.

With regard to the control of polarization direction, Japanese Laid-OpenPatent Application 2001-60739 describes a polarization controltechnology that uses a substrate having a surface orientation inclinedfrom (100) in the direction of (111)A surface of (111)B surface by15°-40° and increases the optical gain in the inclined direction byutilizing the optical gain anisotropy and further by using the multiplequantum well active layer of InAlGaAs and InGaAsP having a compressivestrain. Further, Japanese Laid-Open Patent Application 2001-168461describes the construction of forming the mesa shape in a circularshape, elliptical shape or elongated circular shape and setting thedirection of the major axis from (100) in the direction of (111)Asurface of (111)B surface. In this case, the surface orientation of thesubstrate is inclined from (100) in the [110] direction by 2°(including)-5°-+5°, and the substrate is not the one inclined in the Asurface or B surface direction.

With the surface-emission laser diode according to the seventhconstruction of the twelfth mode of the present invention, the opticalgain anisotropy caused by inclining the substrate orientation in the(111)}A direction is utilized for the polarization control. However,with the present construction, it is not possible to user thepolarization control technology that uses the (311)B substrate andthought most promising at the present juncture. Thus, with the seventhconstruction, the inclination angle of the substrate is smaller (5° to20°) than the (311)B substrate (25°), and the optical gain anisotropyachieved with substrate inclination becomes inevitably small, althoughthere are advantages such as the substrate cost is suppressed and thecleaving process is conducted easily.

Thus, with the seventh construction, this decrease is compensated for bythe increase of optical gain anisotropy obtained by providing acompressive strain to the quantum well active layer.

Thus, the surface-emission laser diode according to the seventhconstruction provides a shape anisotropy with regard to the outer shapeof the active layer as viewed from the optical emission direction, suchthat the direction in the (111)A surface is elongated. Further, theouter shape of the active layer as viewed from the optical emissiondirection of the surface-emission laser diode is provided withanisotropy such that the direction of the (111)A surface is elongated,the optical gain in the inclined direction ((111)A surface direction) isincreased further, and the polarization controllability is improved.

(Thirteenth Mode)

Hereinafter, a surface-emission laser 400 according to a thirteenth modeof the present invention will be explained with reference to FIGS. 17through 19.

The thirteenth mode gives more concrete shape of the first constructionof the twelfth mode, wherein FIG. 17 shows the surface-emission laserdiode 400 in a cross-sectional view for explanation of the principle,while FIG. 18 is a cross-sectional view showing the active layer and itsperiphery of the surface-emission laser diode 400 with enlarged scale.Further, FIG. 19 is a plan view showing a part of the surface-emissionlaser diode 400.

The surface-emission laser diode 400 of the thirteenth aspect of thepresent invention uses an n-(100)GaAs substrate 411 having a surfaceorientation inclined in a direction of (111)A surface with aninclination angle 15°, and on the GaAs substrate 411, there is formed ann-semiconductor distributed Bragg reflector (lower reflector) 414 by aperiodic structure 412 in which an n-Al_(0.9)Ga_(0.1)As layer and ann-Al_(0.3)Ga_(0.7)As layer are stacked alternately for 35 periods withthe thickness corresponding to ¼ times the oscillation wavelength in themedium, wherein there is formed an n-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)Plow refractive index layer 413 on the periodic structure 412 with thethickness of the ¼ wavelength is formed.

Further, there is interposed a compositional graded layer (not shown) ofthe thickness of 20 nm and having the Al content changed gradually fromone value to the other value between the n-Al_(0.9)Ga_(0.1)As layer andthe n-Al_(0.3)Ga_(0.7)As layer constituting one unit of repetition inthe periodic structure 412, wherein the thickness of the foregoing unitincluding the compositional graded layer is set to be equal to ¼ timesthe oscillation wavelength in the medium. With such a construction, theband discontinuity between the high refractive index layer and the lowrefractive index layer is smoothed at the time of causing to flowcurrent to the DBR, and increase of electric resistance can suppressed.

Further, on the periodic structure 412, there are stacked consecutively:an (Al_(0.2)Ga_(0.8))_(0.5)In_(0.5)P lower first spacer layer 415 thatachieves lattice matching to the substrate 411; an active layer 418 inwhich three GaInPAs quantum well active layers 415 having a compressivestrain composition and the bandgap wavelength of 780 nm and fourGa_(0.5)In_(0.5)P barrier layers 417 achieving lattice matching to thesubstrate 411 are stacked alternately, and an(Al_(0.2)Ga_(0.8))_(0.5)In_(0.5)P upper first spacer 419.

Further, on the upper spacer layer 419, there is formed ap-semiconductor distributed Bragg reflector (422) formed of ap-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer (servingalso for the cladding layer) having the thickness of ¼ times the laseroscillation wavelength in that medium and a periodic structure 412 inwhich a p-Al_(x)Ga_(1-x)As (x=0.9) low refractive index layer and ap-Al_(x)Ga_(1-x)As (x=0.3) high refractive index layer are stackedalternately for 24.5 periods, for example (details are omitted in FIG.1). Here, a compositional graded layer similar to the lower reflector414 is interposed between the high refractive index layer and the lowrefractive index layer in the upper reflector 422.

Further, a p-GaAs contact layer 423 is formed on the uppermost part ofthe upper reflector 422 for achieving ohmic contact with the electrode.

With such a construction, there is formed a cavity region 424 of oneoscillation wavelength (so-called lambda cavity) between the lowerreflector 414 and the upper reflector 422. Thereby, the cavity region424 forms, together with the lower reflector 414, the upper reflector424, the cavity structure of the surface-emission laser 400.

With the surface-emission laser diode 400 according to the thirteenthmode of the present invention, the interface 426 (corresponds tointerface 3 of FIG. 15) between the p-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)Plow refractive index layer (functioning also as cladding layer) 420 andthe p-Al_(x)Ga_(1-x)As (x=0.3) 425 formed on the low-refractive indexlayer 450 and constituting the upper reflector 422 is located coincidentto the node of the electric field strength distribution.

Conventionally, the AlGaInP cladding layer (spacer layer) is formed inthe uppermost part of the cavity region. With such a structure, theinterface between the cavity region and the upper reflector of thematerial of the AlGaAs system is formed at the location of the anti-nodewhere the effect of optical absorption is large. However, in the case ofgrowing a semiconductor layer containing Al, Ga and As as the majorcomponents on the semiconductor layer containing Al, In and P as themajor components, there tends to occur segregation of In such as carryover of In, and it has been difficult to suppress the increase of thethreshold value.

According to the thirteenth mode of the present invention in which theinterface 426 is coincident to the node of the electric field strengthdistribution, the effect of optical absorption at the interface 426 isreduced significantly, and it becomes possible to suppress the increaseof the threshold value effectively even when there is caused somesegregation of In in this part. It should be noted that increase of thethreshold value is suppressed further effectively when the segregationof In is suppressed by providing a thin (Al)GaInP between thep-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer 420 andthe p-Al_(x)Ga_(1-x)As (x=0.3) layer 425 as an In segregation suppressorlayer.

Further, it should be noted that, while the thirteenth mode uses the(Al_(0.7)Ga_(0.3))In_(0.5)P layer for the low refractive index layer ofthe lower reflector 414 closest to the active layer 18 in viewpoint ofproviding symmetry to the laser structure, it is possible to use thematerial of AlGaAs system as long as the problem of segregation of In isto be attended.

Hereinafter, the fabrication process of the surface-emission laser diode400 of the thirteenth mode will be explained.

With the present embodiment, the growth of the semiconductor stackedstructure constituting the surface-emission laser diode 400 is conductedby an MOCVD process. Thereby, TMG (trimethyl gallium), TMA (trimethylaluminum), TMI (trimethyl indium), PH₃ (phosphine), and AsH₃ (arsine)are used for the source, and H₂Se (hydrogen selenide) is used for then-type dopant. Further, DMZn or CBr₄ is used for the p-type dopant.Further, H₂ is used for the carrier gas.

Because it is possible to form the construction such as a compositionalgraded layer by controlling the supply amount of the source gases, MOCVDprocess is particularly suitable for the crystal growth method of asurface-emission laser diode that includes DBR. Further, it does notrequire high vacuum state as in the case of MBE process and it issufficient to merely control the supply flow rate of the source gasesand the supply time, MOCVD process is suited for mass production.

With the thirteenth mode, it should be noted that a part of the lowrefractive index layer of the p-side DBR close to the active layer 418is formed by an AlAs layer.

Further, the stacked structure thus formed is subjected to a mesaetching, there is formed a mesa structure 426 of a predetermined sizesuch that at lease a sidewall surface of the p-AlAs layer 427 isexposed. Further, the AlAs layer thus exposed is oxidized in water vaporambient, starting from the exposed sidewall surface, and there is formedan Al_(x)O_(y) current confinement layer 428.

Further, the space surrounding the mesa structure 426 is filled with apolyimide film 432 for planarization, and thereafter, the polyimide film432 is removed from the contact layer 423 in the part corresponding tothe optical exit window 429. Further, a p-side electrode 430 is formedon the p-contact layer 423 so as to avoid the optical exit part 29.Further, an n-side electrode 431 is formed on the rear side of thesubstrate 411.

With the surface emission laser 400 of the present mode, currentconfinement is achieved by selective oxidation of the layer forselective oxidation 427 that contains Al and As as the major components,and with this, threshold current is reduced significantly.

With the current confinement structure that uses an Al oxide film formedby selective oxidation of the layer for selective oxidation 427, itbecomes possible to form the current confinement layer 428 near theactive layer 418, and diffusion of the injected holes is suppressed, andit becomes possible to efficiently confine the carriers into a minuteregion not exposed to the air.

Further, with such a current confinement structure, there occurs adecrease of refractive index when the oxidized part 428 is changed to anAl oxide film, and it becomes possible to confine the light to theminute region where the carriers are confined by way of convex lenseffect. Thereby, a very large efficiency is realized, and with this, thethreshold current is decreased. Further, such a current confinementstructure can be easily formed, and it becomes possible to reduce theproduction cost of the surface-emission laser diode significantly.

Further, with the present mode, the mesa shape as viewed from thedirection of optical emission is formed to have anisotropy in that themesa structure has an elongated elliptic shape in the direction of the(111)A surface as shown in FIG. 19. Such anisotropic shape is notlimited to the elliptic shape, and it is also possible to use othershapes such as a rectangular shape. With this, the current injectionregion formed in the Al oxide film 428 becomes an elongated shape in thedirection of the (111)A surface.

Further, with the surface-emission laser diode 400 of the thirteenthmode, which uses the to material of the AlGaInP system for thelow-refractive index layer of the reflector 422 closest to the activelayer 418 and for the spacer layers 419 and GaInPAs for the barrierlayer 417 or the quantum well active layer 416. Thereby, the laserstructure is formed on the (100) GaAs substrate of the surfaceorientation inclined in the (111)A direction by 15°, and thus, itbecomes possible to suppress the narrowing of bandgap associated withformation of natural superlattices, deterioration of surface morphologycaused by hillock (hill-like defect) formation, or the effect ofnon-optical recombination centers.

Further, with the foregoing construction, it should be noted that(Al_(0.7)Ga)_(0.5)In_(0.5)P, which is a widegap material, is used forthe cladding layer (the low refractive index layer of the reflector 422closest to the active layer 418). Because of this, a very large bandgapdifference of 743 meV is obtained between the cladding layer 420 and theactive layer 418 as compared with the bandgap difference of 466 meV (Alcontent 0.6) for the case the cladding layer 420 is formed of AlGaAs.Similarly, a large bandgap difference is secured between the barrierlayer 417 and the active layer 418, and excellent carrier confinement isrealized. Further, because the active layer 418 accumulates therein acompressive strain, it is also attained the increase of gain caused byband splitting of heavy holes and light holes.

With these effects, the surface-emission laser diode 400 has a very highgain and it is possible to perform high output power operation at lowthreshold.

While it was explained for the case of providing a single layer of(Al_(0.2)Ga_(0.8))_(0.5)In_(0.5)P layer 419 as the spacer layer betweenthe (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer and theactive layer 418, it is also possible to form the space layer by pluralsemiconductor layers. Further, the AlGaInP low refractive index layer420 or the AlGaInP spacer layer may contain other constituent elementswith a trace amount.

Further, because the active layer 418 and the barrier layer 417 areformed of the material free from Al, the active region (quantum wellactive layer and adjacent layers) is free from Al, and incorporation ofoxygen is reduced with the surface-emission laser diode 400. Thereby, itbecomes possible to suppress formation of non-optical recombinationcenters. With this, it becomes possible to realize a surface-emissionlaser diode of long lifetime.

Further, with the surface-emission laser diode 400, control ofpolarization direction is achieved by utilizing the optical gainanisotropy caused by inclination of the substrate 411.

Therefore, with the surface-emission laser diode 400 of the presentembodiment, the inclination angle of the substrate is small (15°) whencompared with the surface-emission laser diode that uses the (311)Bsubstrate (inclination angle 25°), which is thought most promising atthe present juncture.

Thus, with the present mode, this decrease of the optical gainanisotropy is compensated for by the increase of the optical gainanisotropy caused by providing a compressive strain to the quantum wellactive layer 416, and by forming the outer shape of the quantum wellactive layer 416 so as to extend in the direction of the (111)A surfacefor increasing the optical gain in the substrate inclination direction(direction of (111)A). With such a construction, it becomes possible torealize a polarization control not inferior to the case of using the(311)B substrate.

Thus, according to thirteenth mode of the present invention, it becomespossible to realize a high output power surface-emission laser diode of780 nm wavelength band having large gain, low threshold value, goodreliability and controlled polarization direction. Thereby, thesurface-emission laser diode 400 of the present embodiment is designedto the structure in which the increase of threshold value is retardedeven when there is caused segregation of In at the time of conductingcrystal growth of the semiconductor layer 425 containing Al, Ga and Asas the major components on the semiconductor layer 420 that contains Al,In and P as the major components. Thereby, production of the laser diodecan be conducted easily.

It should be noted that the foregoing effect of the present invention ofAl-free active layer decreases at shorter wavelength band, but itappears conspicuously as long as the oscillation wavelength is longerthan 680 nm.

For example, when compared with the surface-emission laser diode of the780 nm band that uses the active layer of AlGaAs/AlGaAs system, itshould be noted that the bandgap difference between the Al_(x)Ga_(1-x)As(x=0.6, Eg=2.0226 eV) having the largest bandgap in the typicalcompositional range used for Al_(x)Ga_(1-x)As (0<x≦1) system spacerlayer used with the surface-emission laser diode of the AlGaAs/AlGaAssystem and the active layer of the compositional wavelength 780 nm(Eg=1.5567 eV), is generally equal to the bandgap difference (460 meV)between the (Al_(a)Ga_(1-a))_(b)In_(1-b)P (a=0.7, b=0.5, Eg=2.269 eV)having the largest bandgap in the typical compositional range used forthe (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) spacer layer and theactive layer of the compositional wavelength of 660 nm (Eg=1.8233 eV).

Further, with regard to the bandgap difference between the barrier layerand the quantum well active layer, the bandgap different to the activelayer of the compositional wavelength of 680 nm becomes about 200 meVwhen the barrier layer is formed of Ga_(e)In_(1-e)P_(f)As_(1-f) (e=0.6,f=1, Eg=2.02 eV), while this is generally equivalent to the case of thesurface-emission laser diode of 780 nm that uses the AlGaAs/AlGaAssystem active layer.

This means that it becomes possible to achieve the carrier confinementequivalent to or superior to the case of the surface-emission laserdiode of the 780 nm band that uses the active layer of the AlGaAs/AlGaAssystem while using an Al-free active layer (quantum well active layerand barrier layer) by using the spacer layer of the AlGaInP system,provided that the compositional wavelength is longer than 680 nm. Infact, the effect of the strained quantum well active layer is added, andit is possible to obtain the characteristics exceeding thesurface-emission laser diode of the AlGaAs/AlGaAs system.

Further, when Al is incorporated to the barrier layer 417, it becomespossible to fabricate a red color surface-emission laser diode of thewavelength shorter than 680 nm such as the device of the 650 nm band. Inthis case, the effect of Al-free active layer is not attained, but itbecomes possible to improve the problem of segregation of In notedbefore.

(Fourteenth Mode)

Next, a fourteenth mode of the present invention will be explained withreference to FIG. 20. It should be noted that the fourteenth moderelates to the construction that gives practical shape to the thirdconstruction mentioned before.

FIG. 20 is a cross-sectional view showing the part around the activelayer of a surface-emission laser diode 500 of the present mode in anenlarged scale.

Referring to FIG. 20, the present embodiment has the constructionsimilar to that of the thirteenth mode of FIG. 18, except that a barrierlayer 417 a of Ga_(0.6)In_(0.4)P having a tensile strain is used inplace of the barrier layer 417.

In the material of the GaInPAs system used for the barrier layer of thequantum well active layer in the surface-emission laser diode, GaInP hasthe largest bandgap when compared with the same lattice constant.Further, a larger bandgap is obtained with the material having a smallerlattice constant. Thus, by using a material of the GaInP system of smalllattice constant for the barrier layer, it is possible to realize alarge band discontinuity between the barrier layer and the quantum wellactive layer, and it becomes possible to increase the gain of thesurface-emission laser diode. With this, the surface-emission laserdiode can perform high output power operation with low threshold value.For example, it should be noted that the bandgap of the tensile-strainedGa_(0.6)In_(0.4)P layer is 2.02 eV, the bandgap of the Ga_(0.5)In_(0.5)Plattice matched layer is 1.87 eV, and thus the Ga_(0.6)In_(0.4)P tensilestrain layer has a bandgap larger by 150 meV.

It should be noted that the effect of increasing the band discontinuityby using a tensile composition layer for the barrier layer is obtainednot only when the quantum well active layer has a compressivecomposition but also in the case it has the lattice-matched compositionor tensile composition.

(Fifteenth Mode)

The surface-emission laser diode according to the fifteenth mode of thepresent invention has a construction similar to that of thesurface-emission laser diode 400 according to the thirteenth modeexplained with reference to FIG. 18, except that C is used for thedopant of the multilayer structure of the p-semiconductor distributedBragg reflector formed of p-Al_(x)Ga_(1-x)As (x=0.9) andp-Al_(x)Ga_(1-x)As (x=0.3) and Mg is used for the dopant of thep-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer.

Zn (zinc) is used extensively for the p-type dopant of (Al)GaInP layer,while Zn has a large diffusion coefficient and easily causes diffusionto the active layer or to the neighbor of the active layer during thegrowth process of the upper reflector. Thereby, there is causeddeterioration of crystal quality in the active layer, resulting indegradation of the device characteristics such as decrease of efficiencyof optical emission or increase of absorption loss. This problem becomesparticularly serious when the p-side semiconductor layer is formedunderside of the active layer.

Further, the surface-emission laser diode has a film thickness severaltimes larger than that of an edge-emission laser diode and thus requireslonger growth time. Thus the problem of thermal diffusion is by no meansa problem that can be neglected.

Mg has a smaller diffusion coefficient than Zn and it is thought thatuse of Mg can improve the foregoing problem. On the other hand, with thesemiconductor layer containing Al, Ga and As as the major components,the diffusion coefficient of C is smaller than the diffusion coefficientof Mg. Further, it is known that controllability becomes poor when Mg isadded to the material of the As system.

Thus, with the present embodiment, Mg is added to the AlGaInP layer andC is added to the AlGaAs multilayer film. With this, the problem ofdopant diffusion or memory effect can be reduced, and it becomespossible to carry out doping with good controllability. Thereby, adoping profile close to the designed profile is obtained, and at thesame time, degradation of the crystal quality of the active layer issuppressed. With this, it becomes possible to achieve low thresholdvalue and high output operation in the surface-emission laser diode.

It should be noted that the effect of using Mg for the dopant of theAlGaInP film and C for the dopant of the AlGaAs film is obtained notonly in the case of using the AlGaInP film as the low refractive indexlayer of the reflector, but also in the case the AlGaInP film isprovided in the cavity region sandwiched between the upper and lowerreflectors as shown in FIG. 21. Further, similar effect is obtained alsoin the case of forming the cavity region by the material of the AlGaInPsystem similarly to the present mode and the reflector is formed of thematerial of the AlGaAs system as in the case of the surface-emissionlaser diode of the 650 nm visible wavelength band.

In the construction of FIG. 21, it should be noted that ann-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P cladding layer 512 is formed on ann-Al_(0.9)Ga_(0.1)As layer 511 constituting the uppermost low refractiveindex layer of the n-type lower reflector, and an active layer is formedon the n-type cladding layer 512 via a (Al_(0.2)Ga_(0.8))_(0.5)In_(0.5)Plower spacer layer 514 in the form of alternate stacking of the GaInPAsquantum active layer 514 accumulating therein a compressive strain and aGa_(0.5)In_(0.5)P barrier layer 515, and wherein there is further formeda p-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P cladding layer 517 on the activelayer via a (Al_(0.2)Ga_(0.8))_(0.5)In_(0.5)P upper spacer layer 516,and the cladding layer 517 is formed with a p-Al_(0.9)Ga_(0.1)As layer518 constituting the lowermost low refractive index layer of the p-typeupper reflector 518.

The n-Al_(0.9)Ga_(0.1)As layer 511 and p-Al_(0.9)Ga_(0.1)As layer 518have the thickness of ¼ times the laser oscillation wavelength in thatmedium, and in the illustrated example, the semiconductor layers 512-517form a cavity 519 of one wavelength between the upper and lowerreflectors 511 and 518.

(Sixteenth Mode)

The surface-emission laser according to sixteenth mode of the presentinvention has a construction similar to that of the surface-emissionlaser diode 400 of FIG. 18, except that stacking of then-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer and thep-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer at thetop part of the lower reflector 413 is replaced with a short periodsuperlattice structure of Al_(0.5)In_(0.5)P and Ga_(0.5)In_(0.5)P.

Although it changes depending on the material, thermal resistance of asemiconductor layer increases with increasing number of the elementsthat constitutes the semiconductor material. Thus, AlGaInP, a quaternarymaterial, has a large thermal resistance. Thus, the heat generated insuch an active layer is not dissipated easily but is accumulated in theactive layer and invites the temperature rise of the active layer. Thus,there has been a problem that the optical output saturates with smallinjection current.

In the case of surface-emission laser diode, it can be regarded, whenthere is formed a superlattice structure by stacking the layers having athickness sufficiently smaller than the oscillating wavelengthalternately, that it is optically equivalent to the case in which thereis formed a mixed crystal having an average composition by mixing theselayers uniformly, although there is a report that there occurs slightincrease of refractive index by forming the superlattice structure.Thus, it is possible to construct a reflector by using the semiconductorlayers thus constructed by the superlattice structure.

Here, in view of the fact that the thermal resistance of the ternarysystem material such as AlInP or GaInP is smaller than that of AlGaInP,a quaternary material, and because of the fact that AlInP or GaInP canbe lattice-matched to the GaAs substrate similarly to the case ofAlGaInP, the present construction decreases the thermal resistance byforming a superlattice structure in place of the conventional AlGaInPsemiconductor layer of uniform composition, by choosing at least twomaterials of small thermal resistance as compared with the averagecomposition. With this, the heat generated in the active layer isefficiently dissipated, and temperature rise of the active layer bycurrent injection is decreased. Further, it becomes possible to conductthe current injection with a higher level than conventional case, whilethis leads to higher output, and it becomes possible to obtain asurface-emission laser diode capable of operating at high output power.

Meanwhile, it should be noted that the effect of thermal resistancereduction for the case of forming AlGaInP by way of short periodsuperlattice structure of AlInP and GaInP is not limited to the case ofusing AlGaInP for the low refractive index layer of a reflector, but itis obtained also for the case of the superlattice structure provided inthe cavity region 519 sandwiched by the upper and lower reflectors 511and 518. Further, similar effect is obtained also in thesurface-emission laser diode that forms the cavity region by thematerial of the AlGaInP system as in the case f the visible 650 nm bandsurface-emission laser diode.

Further, because heat dissipation from the active layer is madeprimarily at the side of the substrate, it is preferable that theforegoing construction is applied at least to the AlGaInP layer at thesubstrate side.

(Seventeenth Mode)

Hereinafter, a surface-emission laser diode 600 according to theseventeenth mode of the present invention will be explained withreference to FIG. 22, wherein those parts of FIG. 222 explainedpreviously are designated by the same reference numerals and thedescription thereof will be omitted.

The surface-emission laser diode 600 according to the present embodimenthas a construction similar to that of the surface-emission laser diode500 according to the fourteenth mode, except for the following twopoints.

The first point is that the lower reflector 414 is formed of consecutivestacking, from the side of the substrate 411, of a first lower reflector412A in which an n-AlAs low refractive index layer and ann-Al_(0.3)Ga_(0.7)As high refractive index layer are stacked for 31periods, and a second lower reflector 412B in which ann-Al_(0.3)Ga_(0.7)As low refractive index layer and ann-Al_(0.3)Ga_(0.7)As high refractive index layer are stacked for 9periods, and that the n-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5) low refractiveindex layer 413 is formed thereon.

Because the lower reflector 414 contains AlAs of small thermalresistance as the low refractive index layer, dissipationcharacteristics of the heat generated in the active layer 416 isimproved and the temperature rise at the time of driving is suppressed.Thereby, a high output power surface-emission laser diode of excellenttemperature characteristics is obtained.

Meanwhile, in the case the etching surface reaches, at the time offormation of the mesa structure, the AlAs layer of the first lowerreflector 412A, there proceeds oxidation also from the edge surface ofthe AlAs layer exposed at the sidewall at the time of oxidationprocessing of the AlAs layer for selective oxidation to be conductedlaser, and there may arise the problem that the active layer 416 isinsulated or electric resistance of the laser diode is increased. Thisis because the oxidation rate of AlGaAs depends heavily on the Alcontent such that the oxidation rate increases with increasing Alcontent and reaches maximum at the composition of AlAs.

Because of this, with the present mode, the second lower reflector 412Bof AlGaAs of small oxidation rate is provided on the foregoing lowerreflector 412A. It should be noted that such second lower reflector 412Bbecomes necessary in the case the oxidation rate of the low refractiveindex layer of the first lower reflector 412A is formed of the materialor thickness faster than the layer for selective oxidation. For example,there are cases in which the lower refractive index of the first lowerreflector 412A is larger than that of the layer for selective oxidation427 even when Ga is contained, as in the case in which the layer forselective oxidation 427 is formed of a material in which a small amountof Ga is added to AlAs. In such a case, too, the second lower reflector412B becomes necessary. As long as the low refractive index layer in thefirst lower reflector 412A has a composition (material) of small thermalresistance as compared with that of the low refractive index layer inthe second lower reflector 412B, efficient heat dissipation is realized.

In the mesa etching, there arises variation in each lot, while in thepresent mode, it is sufficient that the mesa etching is conducted suchthat the etching stops between the layer for selective oxidation 427 ofAl oxide and the AlAs layer 412A of the first lower reflector.

It should be noted that this mesa etching can be conducted by a reactionbeam etching (RIBE) method by introducing a Cl₂ gas into the processingvessel of a dry etching apparatus that holds the substrate to beprocessed. During this mesa etching process, it becomes possible tomonitor the progress of etching by obtaining the ratio of In emission(451 nm) and Al emission (396 nm) are obtained by using a plasma atomicemission spectrometer as explained already with previous embodiment.

After several minutes from the start of the etching, the In emission(451 nm) is detected while the In emission disappears in the meantime.Thus, by terminating the etching at the moment the emission of In hasdisappeared, it becomes possible to stop the etching inside the secondlower reflector 412B. Thereby, it becomes control the mesa etchingeasily with good reproducibility.

The second point is that there is provided a(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P intermediate layer 601 of thethickness of 20 nm, for example, at the interface of theAl_(0.3)Ga_(0.7)As high refractive index layer and the(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer of bothn-side and p-side.

By inserting the intermediate layer 601 of low Al content, it becomespossible with the present mode to expand the range of growth conditionin which planar crystal growth can be conducted with good crystalquality particularly in the case of stacking a p-Al_(0.3)Ga_(0.7)As highrefractive index layer on the p-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P lowrefractive index layer, and formation of the high refractive index layercan be conducted easily. Further, while there arises a problem ofincreased band discontinuity in the valence band at the heterojunctionof a material of the AlGaAs system and a material of the AlGaInP system,it becomes possible to reduce such band discontinuity of the valenceband by inserting such an intermediate layer of low Al content, and itbecomes possible to decrease the electric resistance. Thereby, theintermediate layer 601 may contain As.

Further, by using the surface-emission laser diode of the twelfththrough seventeenth modes of the present invention, it becomes possibleto construct the surface-emission laser diode array 220, the opticaltransmission module 240, the optical communication system 260, the laserprinter 280 that uses the surface-emission laser array chip 281, or thelike.

Further, the present invention is not limited to the embodimentsheretofore but various variations and modifications may be made withinthe scope of the invention described in the patent claims.

1. A surface-emission laser diode, comprising: a GaAs substrate; acavity region formed over said GaAs substrate, said cavity regionincluding at least one quantum well active layer producing a laser lightand harrier layers; and an upper reflector and a lower reflectorprovided at a top part and a bottom part of said cavity region over saidGaAs substrate, said upper reflector and/or lower reflector including asemiconductor distributed Bragg reflector, at least a part of saidsemiconductor distributed Bragg reflector comprising a low refractiveindex layer of Al_(x)Ga_(1-x)As (0<x≦1) and a high refractive indexlayer of Al_(y)Ga_(1-y)As (0≦y>x≦1), one of said low refractive indexlayers constituting said upper reflector and/or said lower reflector andlocated closest to said active layer comprising(Al_(a)Ga_(1-a))_(b)In_(1-b)P(0<a≦1, 0≦b≦1), an interface between saidcavity region and said low refractive index layer of said upperreflector and/or said lower reflector located closest to said activelayer being coincident to an anti-node of an electric strengthdistribution.
 2. The surface-emission laser diode as claimed in claim 1,characterized in that said lower reflector comprises, consecutively fromsaid substrate, a first lower reflector containing an AlAs layer as saidlow refractive index layer, a second lower reflector includingAl_(x1)Ga_(1-x1)As (0<x₁<1) as said low refractive index layer, and atleast one (Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractiveindex layer.
 3. The surface-emission laser diode as claimed in claim 1,characterized in that said high refractive index layer comprisesAl_(y)Ga_(1-y)As (0≦y<x≦1), said low refractive index layer comprises(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1), wherein there is providedan intermediate layer (Al_(a1)Ga_(1-a1))_(b1)In_(1-b1)P (0≦a₁<a≦1,0≦b₁≦1) having an Al content smaller than said(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) low refractive index layerat an interface between said high refractive index layer and said lowrefractive index layer.
 4. The surface-emission laser diode as claimedin claim 1, characterized in that there is provided a spacer layerbetween said active layer and said upper reflector and/or lowerreflector, a part of said spacer layer comprising(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) having a bandgap smallerthan that of said AlGaInP low refractive index layer, said quantum wellactive layer comprising Ga_(c)In_(1-c)P_(d)As_(1-d) (0≦c≦1, 0≦d≦1), saidbarrier layer comprising Ga_(e)In_(1-e)P_(f)As_(1-f) (0≦e≦1, 0≦f≦1). 5.The surface-emission laser as claimed in claim 4, characterized in thatsaid quantum well active layer has a compressive strain.
 6. Thesurface-emission laser diode as claimed in claim 5, characterized inthat said barrier layer has a tensile strain.
 7. The surface-emissionlaser diode as claimed in claim 1, characterized in that said lowrefractive index layer of Al_(x)Ga_(1-x)As (0<x≦1) and said highrefractive index layer of Al_(y)Ga_(1-y)As (0≦y≦x<1) of saidsemiconductor distributed Bragg reflector are doped with a p-type dopantof C (carbon), and wherein said low-refractive index of(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) is doped with a p-typedopant of Mg (magnesium).
 8. The surface-emission laser diode as claimedin claim 1, characterized in that said low-refractive index of(Al_(a)Ga_(1-a))_(b)In_(1-b)P (0<a≦1, 0≦b≦1) is constructed by a shortperiod superlattice structure of AlInP and GaInP.
 9. An image formingapparatus comprising: the surface-emission laser diode as claimed inclaim 1 to generate one or more light beams; a photosensitive body; andan optical system that focuses the one or more light beams upon thephotosensitive body to form an optical image on the photosensitive body.10. An optical transmission apparatus comprising: the surface-emissionlaser diode as claimed in claim 1 to generate one or more light beams;and one or more optical transmission media that carry the respectivelight beams.